x86/gart: Exclude GART aperture from vmcore

[sfrench/cifs-2.6.git] / fs / btrfs / scrub.c

/*
 * Copyright (C) 2011, 2012 STRATO.  All rights reserved.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public
 * License v2 as published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public
 * License along with this program; if not, write to the
 * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
 * Boston, MA 021110-1307, USA.
 */

#include <linux/blkdev.h>
#include <linux/ratelimit.h>
#include <linux/sched/mm.h>
#include "ctree.h"
#include "volumes.h"
#include "disk-io.h"
#include "ordered-data.h"
#include "transaction.h"
#include "backref.h"
#include "extent_io.h"
#include "dev-replace.h"
#include "check-integrity.h"
#include "rcu-string.h"
#include "raid56.h"

/*
 * This is only the first step towards a full-features scrub. It reads all
 * extent and super block and verifies the checksums. In case a bad checksum
 * is found or the extent cannot be read, good data will be written back if
 * any can be found.
 *
 * Future enhancements:
 *  - In case an unrepairable extent is encountered, track which files are
 *    affected and report them
 *  - track and record media errors, throw out bad devices
 *  - add a mode to also read unallocated space
 */

struct scrub_block;
struct scrub_ctx;

/*
 * the following three values only influence the performance.
 * The last one configures the number of parallel and outstanding I/O
 * operations. The first two values configure an upper limit for the number
 * of (dynamically allocated) pages that are added to a bio.
 */
#define SCRUB_PAGES_PER_RD_BIO  32      /* 128k per bio */
#define SCRUB_PAGES_PER_WR_BIO  32      /* 128k per bio */
#define SCRUB_BIOS_PER_SCTX     64      /* 8MB per device in flight */

/*
 * the following value times PAGE_SIZE needs to be large enough to match the
 * largest node/leaf/sector size that shall be supported.
 * Values larger than BTRFS_STRIPE_LEN are not supported.
 */
#define SCRUB_MAX_PAGES_PER_BLOCK       16      /* 64k per node/leaf/sector */

struct scrub_recover {
        refcount_t              refs;
        struct btrfs_bio        *bbio;
        u64                     map_length;
};

struct scrub_page {
        struct scrub_block      *sblock;
        struct page             *page;
        struct btrfs_device     *dev;
        struct list_head        list;
        u64                     flags;  /* extent flags */
        u64                     generation;
        u64                     logical;
        u64                     physical;
        u64                     physical_for_dev_replace;
        atomic_t                refs;
        struct {
                unsigned int    mirror_num:8;
                unsigned int    have_csum:1;
                unsigned int    io_error:1;
        };
        u8                      csum[BTRFS_CSUM_SIZE];

        struct scrub_recover    *recover;
};

struct scrub_bio {
        int                     index;
        struct scrub_ctx        *sctx;
        struct btrfs_device     *dev;
        struct bio              *bio;
        blk_status_t            status;
        u64                     logical;
        u64                     physical;
#if SCRUB_PAGES_PER_WR_BIO >= SCRUB_PAGES_PER_RD_BIO
        struct scrub_page       *pagev[SCRUB_PAGES_PER_WR_BIO];
#else
        struct scrub_page       *pagev[SCRUB_PAGES_PER_RD_BIO];
#endif
        int                     page_count;
        int                     next_free;
        struct btrfs_work       work;
};

struct scrub_block {
        struct scrub_page       *pagev[SCRUB_MAX_PAGES_PER_BLOCK];
        int                     page_count;
        atomic_t                outstanding_pages;
        refcount_t              refs; /* free mem on transition to zero */
        struct scrub_ctx        *sctx;
        struct scrub_parity     *sparity;
        struct {
                unsigned int    header_error:1;
                unsigned int    checksum_error:1;
                unsigned int    no_io_error_seen:1;
                unsigned int    generation_error:1; /* also sets header_error */

                /* The following is for the data used to check parity */
                /* It is for the data with checksum */
                unsigned int    data_corrected:1;
        };
        struct btrfs_work       work;
};

/* Used for the chunks with parity stripe such RAID5/6 */
struct scrub_parity {
        struct scrub_ctx        *sctx;

        struct btrfs_device     *scrub_dev;

        u64                     logic_start;

        u64                     logic_end;

        int                     nsectors;

        u64                     stripe_len;

        refcount_t              refs;

        struct list_head        spages;

        /* Work of parity check and repair */
        struct btrfs_work       work;

        /* Mark the parity blocks which have data */
        unsigned long           *dbitmap;

        /*
         * Mark the parity blocks which have data, but errors happen when
         * read data or check data
         */
        unsigned long           *ebitmap;

        unsigned long           bitmap[0];
};

struct scrub_ctx {
        struct scrub_bio        *bios[SCRUB_BIOS_PER_SCTX];
        struct btrfs_fs_info    *fs_info;
        int                     first_free;
        int                     curr;
        atomic_t                bios_in_flight;
        atomic_t                workers_pending;
        spinlock_t              list_lock;
        wait_queue_head_t       list_wait;
        u16                     csum_size;
        struct list_head        csum_list;
        atomic_t                cancel_req;
        int                     readonly;
        int                     pages_per_rd_bio;

        int                     is_dev_replace;

        struct scrub_bio        *wr_curr_bio;
        struct mutex            wr_lock;
        int                     pages_per_wr_bio; /* <= SCRUB_PAGES_PER_WR_BIO */
        struct btrfs_device     *wr_tgtdev;
        bool                    flush_all_writes;

        /*
         * statistics
         */
        struct btrfs_scrub_progress stat;
        spinlock_t              stat_lock;

        /*
         * Use a ref counter to avoid use-after-free issues. Scrub workers
         * decrement bios_in_flight and workers_pending and then do a wakeup
         * on the list_wait wait queue. We must ensure the main scrub task
         * doesn't free the scrub context before or while the workers are
         * doing the wakeup() call.
         */
        refcount_t              refs;
};

struct scrub_fixup_nodatasum {
        struct scrub_ctx        *sctx;
        struct btrfs_device     *dev;
        u64                     logical;
        struct btrfs_root       *root;
        struct btrfs_work       work;
        int                     mirror_num;
};

struct scrub_nocow_inode {
        u64                     inum;
        u64                     offset;
        u64                     root;
        struct list_head        list;
};

struct scrub_copy_nocow_ctx {
        struct scrub_ctx        *sctx;
        u64                     logical;
        u64                     len;
        int                     mirror_num;
        u64                     physical_for_dev_replace;
        struct list_head        inodes;
        struct btrfs_work       work;
};

struct scrub_warning {
        struct btrfs_path       *path;
        u64                     extent_item_size;
        const char              *errstr;
        u64                     physical;
        u64                     logical;
        struct btrfs_device     *dev;
};

struct full_stripe_lock {
        struct rb_node node;
        u64 logical;
        u64 refs;
        struct mutex mutex;
};

static void scrub_pending_bio_inc(struct scrub_ctx *sctx);
static void scrub_pending_bio_dec(struct scrub_ctx *sctx);
static void scrub_pending_trans_workers_inc(struct scrub_ctx *sctx);
static void scrub_pending_trans_workers_dec(struct scrub_ctx *sctx);
static int scrub_handle_errored_block(struct scrub_block *sblock_to_check);
static int scrub_setup_recheck_block(struct scrub_block *original_sblock,
                                     struct scrub_block *sblocks_for_recheck);
static void scrub_recheck_block(struct btrfs_fs_info *fs_info,
                                struct scrub_block *sblock,
                                int retry_failed_mirror);
static void scrub_recheck_block_checksum(struct scrub_block *sblock);
static int scrub_repair_block_from_good_copy(struct scrub_block *sblock_bad,
                                             struct scrub_block *sblock_good);
static int scrub_repair_page_from_good_copy(struct scrub_block *sblock_bad,
                                            struct scrub_block *sblock_good,
                                            int page_num, int force_write);
static void scrub_write_block_to_dev_replace(struct scrub_block *sblock);
static int scrub_write_page_to_dev_replace(struct scrub_block *sblock,
                                           int page_num);
static int scrub_checksum_data(struct scrub_block *sblock);
static int scrub_checksum_tree_block(struct scrub_block *sblock);
static int scrub_checksum_super(struct scrub_block *sblock);
static void scrub_block_get(struct scrub_block *sblock);
static void scrub_block_put(struct scrub_block *sblock);
static void scrub_page_get(struct scrub_page *spage);
static void scrub_page_put(struct scrub_page *spage);
static void scrub_parity_get(struct scrub_parity *sparity);
static void scrub_parity_put(struct scrub_parity *sparity);
static int scrub_add_page_to_rd_bio(struct scrub_ctx *sctx,
                                    struct scrub_page *spage);
static int scrub_pages(struct scrub_ctx *sctx, u64 logical, u64 len,
                       u64 physical, struct btrfs_device *dev, u64 flags,
                       u64 gen, int mirror_num, u8 *csum, int force,
                       u64 physical_for_dev_replace);
static void scrub_bio_end_io(struct bio *bio);
static void scrub_bio_end_io_worker(struct btrfs_work *work);
static void scrub_block_complete(struct scrub_block *sblock);
static void scrub_remap_extent(struct btrfs_fs_info *fs_info,
                               u64 extent_logical, u64 extent_len,
                               u64 *extent_physical,
                               struct btrfs_device **extent_dev,
                               int *extent_mirror_num);
static int scrub_add_page_to_wr_bio(struct scrub_ctx *sctx,
                                    struct scrub_page *spage);
static void scrub_wr_submit(struct scrub_ctx *sctx);
static void scrub_wr_bio_end_io(struct bio *bio);
static void scrub_wr_bio_end_io_worker(struct btrfs_work *work);
static int write_page_nocow(struct scrub_ctx *sctx,
                            u64 physical_for_dev_replace, struct page *page);
static int copy_nocow_pages_for_inode(u64 inum, u64 offset, u64 root,
                                      struct scrub_copy_nocow_ctx *ctx);
static int copy_nocow_pages(struct scrub_ctx *sctx, u64 logical, u64 len,
                            int mirror_num, u64 physical_for_dev_replace);
static void copy_nocow_pages_worker(struct btrfs_work *work);
static void __scrub_blocked_if_needed(struct btrfs_fs_info *fs_info);
static void scrub_blocked_if_needed(struct btrfs_fs_info *fs_info);
static void scrub_put_ctx(struct scrub_ctx *sctx);


static void scrub_pending_bio_inc(struct scrub_ctx *sctx)
{
        refcount_inc(&sctx->refs);
        atomic_inc(&sctx->bios_in_flight);
}

static void scrub_pending_bio_dec(struct scrub_ctx *sctx)
{
        atomic_dec(&sctx->bios_in_flight);
        wake_up(&sctx->list_wait);
        scrub_put_ctx(sctx);
}

static void __scrub_blocked_if_needed(struct btrfs_fs_info *fs_info)
{
        while (atomic_read(&fs_info->scrub_pause_req)) {
                mutex_unlock(&fs_info->scrub_lock);
                wait_event(fs_info->scrub_pause_wait,
                   atomic_read(&fs_info->scrub_pause_req) == 0);
                mutex_lock(&fs_info->scrub_lock);
        }
}

static void scrub_pause_on(struct btrfs_fs_info *fs_info)
{
        atomic_inc(&fs_info->scrubs_paused);
        wake_up(&fs_info->scrub_pause_wait);
}

static void scrub_pause_off(struct btrfs_fs_info *fs_info)
{
        mutex_lock(&fs_info->scrub_lock);
        __scrub_blocked_if_needed(fs_info);
        atomic_dec(&fs_info->scrubs_paused);
        mutex_unlock(&fs_info->scrub_lock);

        wake_up(&fs_info->scrub_pause_wait);
}

static void scrub_blocked_if_needed(struct btrfs_fs_info *fs_info)
{
        scrub_pause_on(fs_info);
        scrub_pause_off(fs_info);
}

/*
 * Insert new full stripe lock into full stripe locks tree
 *
 * Return pointer to existing or newly inserted full_stripe_lock structure if
 * everything works well.
 * Return ERR_PTR(-ENOMEM) if we failed to allocate memory
 *
 * NOTE: caller must hold full_stripe_locks_root->lock before calling this
 * function
 */
static struct full_stripe_lock *insert_full_stripe_lock(
                struct btrfs_full_stripe_locks_tree *locks_root,
                u64 fstripe_logical)
{
        struct rb_node **p;
        struct rb_node *parent = NULL;
        struct full_stripe_lock *entry;
        struct full_stripe_lock *ret;

        WARN_ON(!mutex_is_locked(&locks_root->lock));

        p = &locks_root->root.rb_node;
        while (*p) {
                parent = *p;
                entry = rb_entry(parent, struct full_stripe_lock, node);
                if (fstripe_logical < entry->logical) {
                        p = &(*p)->rb_left;
                } else if (fstripe_logical > entry->logical) {
                        p = &(*p)->rb_right;
                } else {
                        entry->refs++;
                        return entry;
                }
        }

        /* Insert new lock */
        ret = kmalloc(sizeof(*ret), GFP_KERNEL);
        if (!ret)
                return ERR_PTR(-ENOMEM);
        ret->logical = fstripe_logical;
        ret->refs = 1;
        mutex_init(&ret->mutex);

        rb_link_node(&ret->node, parent, p);
        rb_insert_color(&ret->node, &locks_root->root);
        return ret;
}

/*
 * Search for a full stripe lock of a block group
 *
 * Return pointer to existing full stripe lock if found
 * Return NULL if not found
 */
static struct full_stripe_lock *search_full_stripe_lock(
                struct btrfs_full_stripe_locks_tree *locks_root,
                u64 fstripe_logical)
{
        struct rb_node *node;
        struct full_stripe_lock *entry;

        WARN_ON(!mutex_is_locked(&locks_root->lock));

        node = locks_root->root.rb_node;
        while (node) {
                entry = rb_entry(node, struct full_stripe_lock, node);
                if (fstripe_logical < entry->logical)
                        node = node->rb_left;
                else if (fstripe_logical > entry->logical)
                        node = node->rb_right;
                else
                        return entry;
        }
        return NULL;
}

/*
 * Helper to get full stripe logical from a normal bytenr.
 *
 * Caller must ensure @cache is a RAID56 block group.
 */
static u64 get_full_stripe_logical(struct btrfs_block_group_cache *cache,
                                   u64 bytenr)
{
        u64 ret;

        /*
         * Due to chunk item size limit, full stripe length should not be
         * larger than U32_MAX. Just a sanity check here.
         */
        WARN_ON_ONCE(cache->full_stripe_len >= U32_MAX);

        /*
         * round_down() can only handle power of 2, while RAID56 full
         * stripe length can be 64KiB * n, so we need to manually round down.
         */
        ret = div64_u64(bytenr - cache->key.objectid, cache->full_stripe_len) *
                cache->full_stripe_len + cache->key.objectid;
        return ret;
}

/*
 * Lock a full stripe to avoid concurrency of recovery and read
 *
 * It's only used for profiles with parities (RAID5/6), for other profiles it
 * does nothing.
 *
 * Return 0 if we locked full stripe covering @bytenr, with a mutex held.
 * So caller must call unlock_full_stripe() at the same context.
 *
 * Return <0 if encounters error.
 */
static int lock_full_stripe(struct btrfs_fs_info *fs_info, u64 bytenr,
                            bool *locked_ret)
{
        struct btrfs_block_group_cache *bg_cache;
        struct btrfs_full_stripe_locks_tree *locks_root;
        struct full_stripe_lock *existing;
        u64 fstripe_start;
        int ret = 0;

        *locked_ret = false;
        bg_cache = btrfs_lookup_block_group(fs_info, bytenr);
        if (!bg_cache) {
                ASSERT(0);
                return -ENOENT;
        }

        /* Profiles not based on parity don't need full stripe lock */
        if (!(bg_cache->flags & BTRFS_BLOCK_GROUP_RAID56_MASK))
                goto out;
        locks_root = &bg_cache->full_stripe_locks_root;

        fstripe_start = get_full_stripe_logical(bg_cache, bytenr);

        /* Now insert the full stripe lock */
        mutex_lock(&locks_root->lock);
        existing = insert_full_stripe_lock(locks_root, fstripe_start);
        mutex_unlock(&locks_root->lock);
        if (IS_ERR(existing)) {
                ret = PTR_ERR(existing);
                goto out;
        }
        mutex_lock(&existing->mutex);
        *locked_ret = true;
out:
        btrfs_put_block_group(bg_cache);
        return ret;
}

/*
 * Unlock a full stripe.
 *
 * NOTE: Caller must ensure it's the same context calling corresponding
 * lock_full_stripe().
 *
 * Return 0 if we unlock full stripe without problem.
 * Return <0 for error
 */
static int unlock_full_stripe(struct btrfs_fs_info *fs_info, u64 bytenr,
                              bool locked)
{
        struct btrfs_block_group_cache *bg_cache;
        struct btrfs_full_stripe_locks_tree *locks_root;
        struct full_stripe_lock *fstripe_lock;
        u64 fstripe_start;
        bool freeit = false;
        int ret = 0;

        /* If we didn't acquire full stripe lock, no need to continue */
        if (!locked)
                return 0;

        bg_cache = btrfs_lookup_block_group(fs_info, bytenr);
        if (!bg_cache) {
                ASSERT(0);
                return -ENOENT;
        }
        if (!(bg_cache->flags & BTRFS_BLOCK_GROUP_RAID56_MASK))
                goto out;

        locks_root = &bg_cache->full_stripe_locks_root;
        fstripe_start = get_full_stripe_logical(bg_cache, bytenr);

        mutex_lock(&locks_root->lock);
        fstripe_lock = search_full_stripe_lock(locks_root, fstripe_start);
        /* Unpaired unlock_full_stripe() detected */
        if (!fstripe_lock) {
                WARN_ON(1);
                ret = -ENOENT;
                mutex_unlock(&locks_root->lock);
                goto out;
        }

        if (fstripe_lock->refs == 0) {
                WARN_ON(1);
                btrfs_warn(fs_info, "full stripe lock at %llu refcount underflow",
                        fstripe_lock->logical);
        } else {
                fstripe_lock->refs--;
        }

        if (fstripe_lock->refs == 0) {
                rb_erase(&fstripe_lock->node, &locks_root->root);
                freeit = true;
        }
        mutex_unlock(&locks_root->lock);

        mutex_unlock(&fstripe_lock->mutex);
        if (freeit)
                kfree(fstripe_lock);
out:
        btrfs_put_block_group(bg_cache);
        return ret;
}

/*
 * used for workers that require transaction commits (i.e., for the
 * NOCOW case)
 */
static void scrub_pending_trans_workers_inc(struct scrub_ctx *sctx)
{
        struct btrfs_fs_info *fs_info = sctx->fs_info;

        refcount_inc(&sctx->refs);
        /*
         * increment scrubs_running to prevent cancel requests from
         * completing as long as a worker is running. we must also
         * increment scrubs_paused to prevent deadlocking on pause
         * requests used for transactions commits (as the worker uses a
         * transaction context). it is safe to regard the worker
         * as paused for all matters practical. effectively, we only
         * avoid cancellation requests from completing.
         */
        mutex_lock(&fs_info->scrub_lock);
        atomic_inc(&fs_info->scrubs_running);
        atomic_inc(&fs_info->scrubs_paused);
        mutex_unlock(&fs_info->scrub_lock);

        /*
         * check if @scrubs_running=@scrubs_paused condition
         * inside wait_event() is not an atomic operation.
         * which means we may inc/dec @scrub_running/paused
         * at any time. Let's wake up @scrub_pause_wait as
         * much as we can to let commit transaction blocked less.
         */
        wake_up(&fs_info->scrub_pause_wait);

        atomic_inc(&sctx->workers_pending);
}

/* used for workers that require transaction commits */
static void scrub_pending_trans_workers_dec(struct scrub_ctx *sctx)
{
        struct btrfs_fs_info *fs_info = sctx->fs_info;

        /*
         * see scrub_pending_trans_workers_inc() why we're pretending
         * to be paused in the scrub counters
         */
        mutex_lock(&fs_info->scrub_lock);
        atomic_dec(&fs_info->scrubs_running);
        atomic_dec(&fs_info->scrubs_paused);
        mutex_unlock(&fs_info->scrub_lock);
        atomic_dec(&sctx->workers_pending);
        wake_up(&fs_info->scrub_pause_wait);
        wake_up(&sctx->list_wait);
        scrub_put_ctx(sctx);
}

static void scrub_free_csums(struct scrub_ctx *sctx)
{
        while (!list_empty(&sctx->csum_list)) {
                struct btrfs_ordered_sum *sum;
                sum = list_first_entry(&sctx->csum_list,
                                       struct btrfs_ordered_sum, list);
                list_del(&sum->list);
                kfree(sum);
        }
}

static noinline_for_stack void scrub_free_ctx(struct scrub_ctx *sctx)
{
        int i;

        if (!sctx)
                return;

        /* this can happen when scrub is cancelled */
        if (sctx->curr != -1) {
                struct scrub_bio *sbio = sctx->bios[sctx->curr];

                for (i = 0; i < sbio->page_count; i++) {
                        WARN_ON(!sbio->pagev[i]->page);
                        scrub_block_put(sbio->pagev[i]->sblock);
                }
                bio_put(sbio->bio);
        }

        for (i = 0; i < SCRUB_BIOS_PER_SCTX; ++i) {
                struct scrub_bio *sbio = sctx->bios[i];

                if (!sbio)
                        break;
                kfree(sbio);
        }

        kfree(sctx->wr_curr_bio);
        scrub_free_csums(sctx);
        kfree(sctx);
}

static void scrub_put_ctx(struct scrub_ctx *sctx)
{
        if (refcount_dec_and_test(&sctx->refs))
                scrub_free_ctx(sctx);
}

static noinline_for_stack
struct scrub_ctx *scrub_setup_ctx(struct btrfs_device *dev, int is_dev_replace)
{
        struct scrub_ctx *sctx;
        int             i;
        struct btrfs_fs_info *fs_info = dev->fs_info;

        sctx = kzalloc(sizeof(*sctx), GFP_KERNEL);
        if (!sctx)
                goto nomem;
        refcount_set(&sctx->refs, 1);
        sctx->is_dev_replace = is_dev_replace;
        sctx->pages_per_rd_bio = SCRUB_PAGES_PER_RD_BIO;
        sctx->curr = -1;
        sctx->fs_info = dev->fs_info;
        for (i = 0; i < SCRUB_BIOS_PER_SCTX; ++i) {
                struct scrub_bio *sbio;

                sbio = kzalloc(sizeof(*sbio), GFP_KERNEL);
                if (!sbio)
                        goto nomem;
                sctx->bios[i] = sbio;

                sbio->index = i;
                sbio->sctx = sctx;
                sbio->page_count = 0;
                btrfs_init_work(&sbio->work, btrfs_scrub_helper,
                                scrub_bio_end_io_worker, NULL, NULL);

                if (i != SCRUB_BIOS_PER_SCTX - 1)
                        sctx->bios[i]->next_free = i + 1;
                else
                        sctx->bios[i]->next_free = -1;
        }
        sctx->first_free = 0;
        atomic_set(&sctx->bios_in_flight, 0);
        atomic_set(&sctx->workers_pending, 0);
        atomic_set(&sctx->cancel_req, 0);
        sctx->csum_size = btrfs_super_csum_size(fs_info->super_copy);
        INIT_LIST_HEAD(&sctx->csum_list);

        spin_lock_init(&sctx->list_lock);
        spin_lock_init(&sctx->stat_lock);
        init_waitqueue_head(&sctx->list_wait);

        WARN_ON(sctx->wr_curr_bio != NULL);
        mutex_init(&sctx->wr_lock);
        sctx->wr_curr_bio = NULL;
        if (is_dev_replace) {
                WARN_ON(!fs_info->dev_replace.tgtdev);
                sctx->pages_per_wr_bio = SCRUB_PAGES_PER_WR_BIO;
                sctx->wr_tgtdev = fs_info->dev_replace.tgtdev;
                sctx->flush_all_writes = false;
        }

        return sctx;

nomem:
        scrub_free_ctx(sctx);
        return ERR_PTR(-ENOMEM);
}

static int scrub_print_warning_inode(u64 inum, u64 offset, u64 root,
                                     void *warn_ctx)
{
        u64 isize;
        u32 nlink;
        int ret;
        int i;
        unsigned nofs_flag;
        struct extent_buffer *eb;
        struct btrfs_inode_item *inode_item;
        struct scrub_warning *swarn = warn_ctx;
        struct btrfs_fs_info *fs_info = swarn->dev->fs_info;
        struct inode_fs_paths *ipath = NULL;
        struct btrfs_root *local_root;
        struct btrfs_key root_key;
        struct btrfs_key key;

        root_key.objectid = root;
        root_key.type = BTRFS_ROOT_ITEM_KEY;
        root_key.offset = (u64)-1;
        local_root = btrfs_read_fs_root_no_name(fs_info, &root_key);
        if (IS_ERR(local_root)) {
                ret = PTR_ERR(local_root);
                goto err;
        }

        /*
         * this makes the path point to (inum INODE_ITEM ioff)
         */
        key.objectid = inum;
        key.type = BTRFS_INODE_ITEM_KEY;
        key.offset = 0;

        ret = btrfs_search_slot(NULL, local_root, &key, swarn->path, 0, 0);
        if (ret) {
                btrfs_release_path(swarn->path);
                goto err;
        }

        eb = swarn->path->nodes[0];
        inode_item = btrfs_item_ptr(eb, swarn->path->slots[0],
                                        struct btrfs_inode_item);
        isize = btrfs_inode_size(eb, inode_item);
        nlink = btrfs_inode_nlink(eb, inode_item);
        btrfs_release_path(swarn->path);

        /*
         * init_path might indirectly call vmalloc, or use GFP_KERNEL. Scrub
         * uses GFP_NOFS in this context, so we keep it consistent but it does
         * not seem to be strictly necessary.
         */
        nofs_flag = memalloc_nofs_save();
        ipath = init_ipath(4096, local_root, swarn->path);
        memalloc_nofs_restore(nofs_flag);
        if (IS_ERR(ipath)) {
                ret = PTR_ERR(ipath);
                ipath = NULL;
                goto err;
        }
        ret = paths_from_inode(inum, ipath);

        if (ret < 0)
                goto err;

        /*
         * we deliberately ignore the bit ipath might have been too small to
         * hold all of the paths here
         */
        for (i = 0; i < ipath->fspath->elem_cnt; ++i)
                btrfs_warn_in_rcu(fs_info,
"%s at logical %llu on dev %s, physical %llu, root %llu, inode %llu, offset %llu, length %llu, links %u (path: %s)",
                                  swarn->errstr, swarn->logical,
                                  rcu_str_deref(swarn->dev->name),
                                  swarn->physical,
                                  root, inum, offset,
                                  min(isize - offset, (u64)PAGE_SIZE), nlink,
                                  (char *)(unsigned long)ipath->fspath->val[i]);

        free_ipath(ipath);
        return 0;

err:
        btrfs_warn_in_rcu(fs_info,
                          "%s at logical %llu on dev %s, physical %llu, root %llu, inode %llu, offset %llu: path resolving failed with ret=%d",
                          swarn->errstr, swarn->logical,
                          rcu_str_deref(swarn->dev->name),
                          swarn->physical,
                          root, inum, offset, ret);

        free_ipath(ipath);
        return 0;
}

static void scrub_print_warning(const char *errstr, struct scrub_block *sblock)
{
        struct btrfs_device *dev;
        struct btrfs_fs_info *fs_info;
        struct btrfs_path *path;
        struct btrfs_key found_key;
        struct extent_buffer *eb;
        struct btrfs_extent_item *ei;
        struct scrub_warning swarn;
        unsigned long ptr = 0;
        u64 extent_item_pos;
        u64 flags = 0;
        u64 ref_root;
        u32 item_size;
        u8 ref_level = 0;
        int ret;

        WARN_ON(sblock->page_count < 1);
        dev = sblock->pagev[0]->dev;
        fs_info = sblock->sctx->fs_info;

        path = btrfs_alloc_path();
        if (!path)
                return;

        swarn.physical = sblock->pagev[0]->physical;
        swarn.logical = sblock->pagev[0]->logical;
        swarn.errstr = errstr;
        swarn.dev = NULL;

        ret = extent_from_logical(fs_info, swarn.logical, path, &found_key,
                                  &flags);
        if (ret < 0)
                goto out;

        extent_item_pos = swarn.logical - found_key.objectid;
        swarn.extent_item_size = found_key.offset;

        eb = path->nodes[0];
        ei = btrfs_item_ptr(eb, path->slots[0], struct btrfs_extent_item);
        item_size = btrfs_item_size_nr(eb, path->slots[0]);

        if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK) {
                do {
                        ret = tree_backref_for_extent(&ptr, eb, &found_key, ei,
                                                      item_size, &ref_root,
                                                      &ref_level);
                        btrfs_warn_in_rcu(fs_info,
"%s at logical %llu on dev %s, physical %llu: metadata %s (level %d) in tree %llu",
                                errstr, swarn.logical,
                                rcu_str_deref(dev->name),
                                swarn.physical,
                                ref_level ? "node" : "leaf",
                                ret < 0 ? -1 : ref_level,
                                ret < 0 ? -1 : ref_root);
                } while (ret != 1);
                btrfs_release_path(path);
        } else {
                btrfs_release_path(path);
                swarn.path = path;
                swarn.dev = dev;
                iterate_extent_inodes(fs_info, found_key.objectid,
                                        extent_item_pos, 1,
                                        scrub_print_warning_inode, &swarn, false);
        }

out:
        btrfs_free_path(path);
}

static int scrub_fixup_readpage(u64 inum, u64 offset, u64 root, void *fixup_ctx)
{
        struct page *page = NULL;
        unsigned long index;
        struct scrub_fixup_nodatasum *fixup = fixup_ctx;
        int ret;
        int corrected = 0;
        struct btrfs_key key;
        struct inode *inode = NULL;
        struct btrfs_fs_info *fs_info;
        u64 end = offset + PAGE_SIZE - 1;
        struct btrfs_root *local_root;
        int srcu_index;

        key.objectid = root;
        key.type = BTRFS_ROOT_ITEM_KEY;
        key.offset = (u64)-1;

        fs_info = fixup->root->fs_info;
        srcu_index = srcu_read_lock(&fs_info->subvol_srcu);

        local_root = btrfs_read_fs_root_no_name(fs_info, &key);
        if (IS_ERR(local_root)) {
                srcu_read_unlock(&fs_info->subvol_srcu, srcu_index);
                return PTR_ERR(local_root);
        }

        key.type = BTRFS_INODE_ITEM_KEY;
        key.objectid = inum;
        key.offset = 0;
        inode = btrfs_iget(fs_info->sb, &key, local_root, NULL);
        srcu_read_unlock(&fs_info->subvol_srcu, srcu_index);
        if (IS_ERR(inode))
                return PTR_ERR(inode);

        index = offset >> PAGE_SHIFT;

        page = find_or_create_page(inode->i_mapping, index, GFP_NOFS);
        if (!page) {
                ret = -ENOMEM;
                goto out;
        }

        if (PageUptodate(page)) {
                if (PageDirty(page)) {
                        /*
                         * we need to write the data to the defect sector. the
                         * data that was in that sector is not in memory,
                         * because the page was modified. we must not write the
                         * modified page to that sector.
                         *
                         * TODO: what could be done here: wait for the delalloc
                         *       runner to write out that page (might involve
                         *       COW) and see whether the sector is still
                         *       referenced afterwards.
                         *
                         * For the meantime, we'll treat this error
                         * incorrectable, although there is a chance that a
                         * later scrub will find the bad sector again and that
                         * there's no dirty page in memory, then.
                         */
                        ret = -EIO;
                        goto out;
                }
                ret = repair_io_failure(fs_info, inum, offset, PAGE_SIZE,
                                        fixup->logical, page,
                                        offset - page_offset(page),
                                        fixup->mirror_num);
                unlock_page(page);
                corrected = !ret;
        } else {
                /*
                 * we need to get good data first. the general readpage path
                 * will call repair_io_failure for us, we just have to make
                 * sure we read the bad mirror.
                 */
                ret = set_extent_bits(&BTRFS_I(inode)->io_tree, offset, end,
                                        EXTENT_DAMAGED);
                if (ret) {
                        /* set_extent_bits should give proper error */
                        WARN_ON(ret > 0);
                        if (ret > 0)
                                ret = -EFAULT;
                        goto out;
                }

                ret = extent_read_full_page(&BTRFS_I(inode)->io_tree, page,
                                                btrfs_get_extent,
                                                fixup->mirror_num);
                wait_on_page_locked(page);

                corrected = !test_range_bit(&BTRFS_I(inode)->io_tree, offset,
                                                end, EXTENT_DAMAGED, 0, NULL);
                if (!corrected)
                        clear_extent_bits(&BTRFS_I(inode)->io_tree, offset, end,
                                                EXTENT_DAMAGED);
        }

out:
        if (page)
                put_page(page);

        iput(inode);

        if (ret < 0)
                return ret;

        if (ret == 0 && corrected) {
                /*
                 * we only need to call readpage for one of the inodes belonging
                 * to this extent. so make iterate_extent_inodes stop
                 */
                return 1;
        }

        return -EIO;
}

static void scrub_fixup_nodatasum(struct btrfs_work *work)
{
        struct btrfs_fs_info *fs_info;
        int ret;
        struct scrub_fixup_nodatasum *fixup;
        struct scrub_ctx *sctx;
        struct btrfs_trans_handle *trans = NULL;
        struct btrfs_path *path;
        int uncorrectable = 0;

        fixup = container_of(work, struct scrub_fixup_nodatasum, work);
        sctx = fixup->sctx;
        fs_info = fixup->root->fs_info;

        path = btrfs_alloc_path();
        if (!path) {
                spin_lock(&sctx->stat_lock);
                ++sctx->stat.malloc_errors;
                spin_unlock(&sctx->stat_lock);
                uncorrectable = 1;
                goto out;
        }

        trans = btrfs_join_transaction(fixup->root);
        if (IS_ERR(trans)) {
                uncorrectable = 1;
                goto out;
        }

        /*
         * the idea is to trigger a regular read through the standard path. we
         * read a page from the (failed) logical address by specifying the
         * corresponding copynum of the failed sector. thus, that readpage is
         * expected to fail.
         * that is the point where on-the-fly error correction will kick in
         * (once it's finished) and rewrite the failed sector if a good copy
         * can be found.
         */
        ret = iterate_inodes_from_logical(fixup->logical, fs_info, path,
                                          scrub_fixup_readpage, fixup, false);
        if (ret < 0) {
                uncorrectable = 1;
                goto out;
        }
        WARN_ON(ret != 1);

        spin_lock(&sctx->stat_lock);
        ++sctx->stat.corrected_errors;
        spin_unlock(&sctx->stat_lock);

out:
        if (trans && !IS_ERR(trans))
                btrfs_end_transaction(trans);
        if (uncorrectable) {
                spin_lock(&sctx->stat_lock);
                ++sctx->stat.uncorrectable_errors;
                spin_unlock(&sctx->stat_lock);
                btrfs_dev_replace_stats_inc(
                        &fs_info->dev_replace.num_uncorrectable_read_errors);
                btrfs_err_rl_in_rcu(fs_info,
                    "unable to fixup (nodatasum) error at logical %llu on dev %s",
                        fixup->logical, rcu_str_deref(fixup->dev->name));
        }

        btrfs_free_path(path);
        kfree(fixup);

        scrub_pending_trans_workers_dec(sctx);
}

static inline void scrub_get_recover(struct scrub_recover *recover)
{
        refcount_inc(&recover->refs);
}

static inline void scrub_put_recover(struct btrfs_fs_info *fs_info,
                                     struct scrub_recover *recover)
{
        if (refcount_dec_and_test(&recover->refs)) {
                btrfs_bio_counter_dec(fs_info);
                btrfs_put_bbio(recover->bbio);
                kfree(recover);
        }
}

/*
 * scrub_handle_errored_block gets called when either verification of the
 * pages failed or the bio failed to read, e.g. with EIO. In the latter
 * case, this function handles all pages in the bio, even though only one
 * may be bad.
 * The goal of this function is to repair the errored block by using the
 * contents of one of the mirrors.
 */
static int scrub_handle_errored_block(struct scrub_block *sblock_to_check)
{
        struct scrub_ctx *sctx = sblock_to_check->sctx;
        struct btrfs_device *dev;
        struct btrfs_fs_info *fs_info;
        u64 length;
        u64 logical;
        unsigned int failed_mirror_index;
        unsigned int is_metadata;
        unsigned int have_csum;
        struct scrub_block *sblocks_for_recheck; /* holds one for each mirror */
        struct scrub_block *sblock_bad;
        int ret;
        int mirror_index;
        int page_num;
        int success;
        bool full_stripe_locked;
        static DEFINE_RATELIMIT_STATE(_rs, DEFAULT_RATELIMIT_INTERVAL,
                                      DEFAULT_RATELIMIT_BURST);

        BUG_ON(sblock_to_check->page_count < 1);
        fs_info = sctx->fs_info;
        if (sblock_to_check->pagev[0]->flags & BTRFS_EXTENT_FLAG_SUPER) {
                /*
                 * if we find an error in a super block, we just report it.
                 * They will get written with the next transaction commit
                 * anyway
                 */
                spin_lock(&sctx->stat_lock);
                ++sctx->stat.super_errors;
                spin_unlock(&sctx->stat_lock);
                return 0;
        }
        length = sblock_to_check->page_count * PAGE_SIZE;
        logical = sblock_to_check->pagev[0]->logical;
        BUG_ON(sblock_to_check->pagev[0]->mirror_num < 1);
        failed_mirror_index = sblock_to_check->pagev[0]->mirror_num - 1;
        is_metadata = !(sblock_to_check->pagev[0]->flags &
                        BTRFS_EXTENT_FLAG_DATA);
        have_csum = sblock_to_check->pagev[0]->have_csum;
        dev = sblock_to_check->pagev[0]->dev;

        /*
         * For RAID5/6, race can happen for a different device scrub thread.
         * For data corruption, Parity and Data threads will both try
         * to recovery the data.
         * Race can lead to doubly added csum error, or even unrecoverable
         * error.
         */
        ret = lock_full_stripe(fs_info, logical, &full_stripe_locked);
        if (ret < 0) {
                spin_lock(&sctx->stat_lock);
                if (ret == -ENOMEM)
                        sctx->stat.malloc_errors++;
                sctx->stat.read_errors++;
                sctx->stat.uncorrectable_errors++;
                spin_unlock(&sctx->stat_lock);
                return ret;
        }

        if (sctx->is_dev_replace && !is_metadata && !have_csum) {
                sblocks_for_recheck = NULL;
                goto nodatasum_case;
        }

        /*
         * read all mirrors one after the other. This includes to
         * re-read the extent or metadata block that failed (that was
         * the cause that this fixup code is called) another time,
         * page by page this time in order to know which pages
         * caused I/O errors and which ones are good (for all mirrors).
         * It is the goal to handle the situation when more than one
         * mirror contains I/O errors, but the errors do not
         * overlap, i.e. the data can be repaired by selecting the
         * pages from those mirrors without I/O error on the
         * particular pages. One example (with blocks >= 2 * PAGE_SIZE)
         * would be that mirror #1 has an I/O error on the first page,
         * the second page is good, and mirror #2 has an I/O error on
         * the second page, but the first page is good.
         * Then the first page of the first mirror can be repaired by
         * taking the first page of the second mirror, and the
         * second page of the second mirror can be repaired by
         * copying the contents of the 2nd page of the 1st mirror.
         * One more note: if the pages of one mirror contain I/O
         * errors, the checksum cannot be verified. In order to get
         * the best data for repairing, the first attempt is to find
         * a mirror without I/O errors and with a validated checksum.
         * Only if this is not possible, the pages are picked from
         * mirrors with I/O errors without considering the checksum.
         * If the latter is the case, at the end, the checksum of the
         * repaired area is verified in order to correctly maintain
         * the statistics.
         */

        sblocks_for_recheck = kcalloc(BTRFS_MAX_MIRRORS,
                                      sizeof(*sblocks_for_recheck), GFP_NOFS);
        if (!sblocks_for_recheck) {
                spin_lock(&sctx->stat_lock);
                sctx->stat.malloc_errors++;
                sctx->stat.read_errors++;
                sctx->stat.uncorrectable_errors++;
                spin_unlock(&sctx->stat_lock);
                btrfs_dev_stat_inc_and_print(dev, BTRFS_DEV_STAT_READ_ERRS);
                goto out;
        }

        /* setup the context, map the logical blocks and alloc the pages */
        ret = scrub_setup_recheck_block(sblock_to_check, sblocks_for_recheck);
        if (ret) {
                spin_lock(&sctx->stat_lock);
                sctx->stat.read_errors++;
                sctx->stat.uncorrectable_errors++;
                spin_unlock(&sctx->stat_lock);
                btrfs_dev_stat_inc_and_print(dev, BTRFS_DEV_STAT_READ_ERRS);
                goto out;
        }
        BUG_ON(failed_mirror_index >= BTRFS_MAX_MIRRORS);
        sblock_bad = sblocks_for_recheck + failed_mirror_index;

        /* build and submit the bios for the failed mirror, check checksums */
        scrub_recheck_block(fs_info, sblock_bad, 1);

        if (!sblock_bad->header_error && !sblock_bad->checksum_error &&
            sblock_bad->no_io_error_seen) {
                /*
                 * the error disappeared after reading page by page, or
                 * the area was part of a huge bio and other parts of the
                 * bio caused I/O errors, or the block layer merged several
                 * read requests into one and the error is caused by a
                 * different bio (usually one of the two latter cases is
                 * the cause)
                 */
                spin_lock(&sctx->stat_lock);
                sctx->stat.unverified_errors++;
                sblock_to_check->data_corrected = 1;
                spin_unlock(&sctx->stat_lock);

                if (sctx->is_dev_replace)
                        scrub_write_block_to_dev_replace(sblock_bad);
                goto out;
        }

        if (!sblock_bad->no_io_error_seen) {
                spin_lock(&sctx->stat_lock);
                sctx->stat.read_errors++;
                spin_unlock(&sctx->stat_lock);
                if (__ratelimit(&_rs))
                        scrub_print_warning("i/o error", sblock_to_check);
                btrfs_dev_stat_inc_and_print(dev, BTRFS_DEV_STAT_READ_ERRS);
        } else if (sblock_bad->checksum_error) {
                spin_lock(&sctx->stat_lock);
                sctx->stat.csum_errors++;
                spin_unlock(&sctx->stat_lock);
                if (__ratelimit(&_rs))
                        scrub_print_warning("checksum error", sblock_to_check);
                btrfs_dev_stat_inc_and_print(dev,
                                             BTRFS_DEV_STAT_CORRUPTION_ERRS);
        } else if (sblock_bad->header_error) {
                spin_lock(&sctx->stat_lock);
                sctx->stat.verify_errors++;
                spin_unlock(&sctx->stat_lock);
                if (__ratelimit(&_rs))
                        scrub_print_warning("checksum/header error",
                                            sblock_to_check);
                if (sblock_bad->generation_error)
                        btrfs_dev_stat_inc_and_print(dev,
                                BTRFS_DEV_STAT_GENERATION_ERRS);
                else
                        btrfs_dev_stat_inc_and_print(dev,
                                BTRFS_DEV_STAT_CORRUPTION_ERRS);
        }

        if (sctx->readonly) {
                ASSERT(!sctx->is_dev_replace);
                goto out;
        }

        if (!is_metadata && !have_csum) {
                struct scrub_fixup_nodatasum *fixup_nodatasum;

                WARN_ON(sctx->is_dev_replace);

nodatasum_case:

                /*
                 * !is_metadata and !have_csum, this means that the data
                 * might not be COWed, that it might be modified
                 * concurrently. The general strategy to work on the
                 * commit root does not help in the case when COW is not
                 * used.
                 */
                fixup_nodatasum = kzalloc(sizeof(*fixup_nodatasum), GFP_NOFS);
                if (!fixup_nodatasum)
                        goto did_not_correct_error;
                fixup_nodatasum->sctx = sctx;
                fixup_nodatasum->dev = dev;
                fixup_nodatasum->logical = logical;
                fixup_nodatasum->root = fs_info->extent_root;
                fixup_nodatasum->mirror_num = failed_mirror_index + 1;
                scrub_pending_trans_workers_inc(sctx);
                btrfs_init_work(&fixup_nodatasum->work, btrfs_scrub_helper,
                                scrub_fixup_nodatasum, NULL, NULL);
                btrfs_queue_work(fs_info->scrub_workers,
                                 &fixup_nodatasum->work);
                goto out;
        }

        /*
         * now build and submit the bios for the other mirrors, check
         * checksums.
         * First try to pick the mirror which is completely without I/O
         * errors and also does not have a checksum error.
         * If one is found, and if a checksum is present, the full block
         * that is known to contain an error is rewritten. Afterwards
         * the block is known to be corrected.
         * If a mirror is found which is completely correct, and no
         * checksum is present, only those pages are rewritten that had
         * an I/O error in the block to be repaired, since it cannot be
         * determined, which copy of the other pages is better (and it
         * could happen otherwise that a correct page would be
         * overwritten by a bad one).
         */
        for (mirror_index = 0;
             mirror_index < BTRFS_MAX_MIRRORS &&
             sblocks_for_recheck[mirror_index].page_count > 0;
             mirror_index++) {
                struct scrub_block *sblock_other;

                if (mirror_index == failed_mirror_index)
                        continue;
                sblock_other = sblocks_for_recheck + mirror_index;

                /* build and submit the bios, check checksums */
                scrub_recheck_block(fs_info, sblock_other, 0);

                if (!sblock_other->header_error &&
                    !sblock_other->checksum_error &&
                    sblock_other->no_io_error_seen) {
                        if (sctx->is_dev_replace) {
                                scrub_write_block_to_dev_replace(sblock_other);
                                goto corrected_error;
                        } else {
                                ret = scrub_repair_block_from_good_copy(
                                                sblock_bad, sblock_other);
                                if (!ret)
                                        goto corrected_error;
                        }
                }
        }

        if (sblock_bad->no_io_error_seen && !sctx->is_dev_replace)
                goto did_not_correct_error;

        /*
         * In case of I/O errors in the area that is supposed to be
         * repaired, continue by picking good copies of those pages.
         * Select the good pages from mirrors to rewrite bad pages from
         * the area to fix. Afterwards verify the checksum of the block
         * that is supposed to be repaired. This verification step is
         * only done for the purpose of statistic counting and for the
         * final scrub report, whether errors remain.
         * A perfect algorithm could make use of the checksum and try
         * all possible combinations of pages from the different mirrors
         * until the checksum verification succeeds. For example, when
         * the 2nd page of mirror #1 faces I/O errors, and the 2nd page
         * of mirror #2 is readable but the final checksum test fails,
         * then the 2nd page of mirror #3 could be tried, whether now
         * the final checksum succeeds. But this would be a rare
         * exception and is therefore not implemented. At least it is
         * avoided that the good copy is overwritten.
         * A more useful improvement would be to pick the sectors
         * without I/O error based on sector sizes (512 bytes on legacy
         * disks) instead of on PAGE_SIZE. Then maybe 512 byte of one
         * mirror could be repaired by taking 512 byte of a different
         * mirror, even if other 512 byte sectors in the same PAGE_SIZE
         * area are unreadable.
         */
        success = 1;
        for (page_num = 0; page_num < sblock_bad->page_count;
             page_num++) {
                struct scrub_page *page_bad = sblock_bad->pagev[page_num];
                struct scrub_block *sblock_other = NULL;

                /* skip no-io-error page in scrub */
                if (!page_bad->io_error && !sctx->is_dev_replace)
                        continue;

                /* try to find no-io-error page in mirrors */
                if (page_bad->io_error) {
                        for (mirror_index = 0;
                             mirror_index < BTRFS_MAX_MIRRORS &&
                             sblocks_for_recheck[mirror_index].page_count > 0;
                             mirror_index++) {
                                if (!sblocks_for_recheck[mirror_index].
                                    pagev[page_num]->io_error) {
                                        sblock_other = sblocks_for_recheck +
                                                       mirror_index;
                                        break;
                                }
                        }
                        if (!sblock_other)
                                success = 0;
                }

                if (sctx->is_dev_replace) {
                        /*
                         * did not find a mirror to fetch the page
                         * from. scrub_write_page_to_dev_replace()
                         * handles this case (page->io_error), by
                         * filling the block with zeros before
                         * submitting the write request
                         */
                        if (!sblock_other)
                                sblock_other = sblock_bad;

                        if (scrub_write_page_to_dev_replace(sblock_other,
                                                            page_num) != 0) {
                                btrfs_dev_replace_stats_inc(
                                        &fs_info->dev_replace.num_write_errors);
                                success = 0;
                        }
                } else if (sblock_other) {
                        ret = scrub_repair_page_from_good_copy(sblock_bad,
                                                               sblock_other,
                                                               page_num, 0);
                        if (0 == ret)
                                page_bad->io_error = 0;
                        else
                                success = 0;
                }
        }

        if (success && !sctx->is_dev_replace) {
                if (is_metadata || have_csum) {
                        /*
                         * need to verify the checksum now that all
                         * sectors on disk are repaired (the write
                         * request for data to be repaired is on its way).
                         * Just be lazy and use scrub_recheck_block()
                         * which re-reads the data before the checksum
                         * is verified, but most likely the data comes out
                         * of the page cache.
                         */
                        scrub_recheck_block(fs_info, sblock_bad, 1);
                        if (!sblock_bad->header_error &&
                            !sblock_bad->checksum_error &&
                            sblock_bad->no_io_error_seen)
                                goto corrected_error;
                        else
                                goto did_not_correct_error;
                } else {
corrected_error:
                        spin_lock(&sctx->stat_lock);
                        sctx->stat.corrected_errors++;
                        sblock_to_check->data_corrected = 1;
                        spin_unlock(&sctx->stat_lock);
                        btrfs_err_rl_in_rcu(fs_info,
                                "fixed up error at logical %llu on dev %s",
                                logical, rcu_str_deref(dev->name));
                }
        } else {
did_not_correct_error:
                spin_lock(&sctx->stat_lock);
                sctx->stat.uncorrectable_errors++;
                spin_unlock(&sctx->stat_lock);
                btrfs_err_rl_in_rcu(fs_info,
                        "unable to fixup (regular) error at logical %llu on dev %s",
                        logical, rcu_str_deref(dev->name));
        }

out:
        if (sblocks_for_recheck) {
                for (mirror_index = 0; mirror_index < BTRFS_MAX_MIRRORS;
                     mirror_index++) {
                        struct scrub_block *sblock = sblocks_for_recheck +
                                                     mirror_index;
                        struct scrub_recover *recover;
                        int page_index;

                        for (page_index = 0; page_index < sblock->page_count;
                             page_index++) {
                                sblock->pagev[page_index]->sblock = NULL;
                                recover = sblock->pagev[page_index]->recover;
                                if (recover) {
                                        scrub_put_recover(fs_info, recover);
                                        sblock->pagev[page_index]->recover =
                                                                        NULL;
                                }
                                scrub_page_put(sblock->pagev[page_index]);
                        }
                }
                kfree(sblocks_for_recheck);
        }

        ret = unlock_full_stripe(fs_info, logical, full_stripe_locked);
        if (ret < 0)
                return ret;
        return 0;
}

static inline int scrub_nr_raid_mirrors(struct btrfs_bio *bbio)
{
        if (bbio->map_type & BTRFS_BLOCK_GROUP_RAID5)
                return 2;
        else if (bbio->map_type & BTRFS_BLOCK_GROUP_RAID6)
                return 3;
        else
                return (int)bbio->num_stripes;
}

static inline void scrub_stripe_index_and_offset(u64 logical, u64 map_type,
                                                 u64 *raid_map,
                                                 u64 mapped_length,
                                                 int nstripes, int mirror,
                                                 int *stripe_index,
                                                 u64 *stripe_offset)
{
        int i;

        if (map_type & BTRFS_BLOCK_GROUP_RAID56_MASK) {
                /* RAID5/6 */
                for (i = 0; i < nstripes; i++) {
                        if (raid_map[i] == RAID6_Q_STRIPE ||
                            raid_map[i] == RAID5_P_STRIPE)
                                continue;

                        if (logical >= raid_map[i] &&
                            logical < raid_map[i] + mapped_length)
                                break;
                }

                *stripe_index = i;
                *stripe_offset = logical - raid_map[i];
        } else {
                /* The other RAID type */
                *stripe_index = mirror;
                *stripe_offset = 0;
        }
}

static int scrub_setup_recheck_block(struct scrub_block *original_sblock,
                                     struct scrub_block *sblocks_for_recheck)
{
        struct scrub_ctx *sctx = original_sblock->sctx;
        struct btrfs_fs_info *fs_info = sctx->fs_info;
        u64 length = original_sblock->page_count * PAGE_SIZE;
        u64 logical = original_sblock->pagev[0]->logical;
        u64 generation = original_sblock->pagev[0]->generation;
        u64 flags = original_sblock->pagev[0]->flags;
        u64 have_csum = original_sblock->pagev[0]->have_csum;
        struct scrub_recover *recover;
        struct btrfs_bio *bbio;
        u64 sublen;
        u64 mapped_length;
        u64 stripe_offset;
        int stripe_index;
        int page_index = 0;
        int mirror_index;
        int nmirrors;
        int ret;

        /*
         * note: the two members refs and outstanding_pages
         * are not used (and not set) in the blocks that are used for
         * the recheck procedure
         */

        while (length > 0) {
                sublen = min_t(u64, length, PAGE_SIZE);
                mapped_length = sublen;
                bbio = NULL;

                /*
                 * with a length of PAGE_SIZE, each returned stripe
                 * represents one mirror
                 */
                btrfs_bio_counter_inc_blocked(fs_info);
                ret = btrfs_map_sblock(fs_info, BTRFS_MAP_GET_READ_MIRRORS,
                                logical, &mapped_length, &bbio);
                if (ret || !bbio || mapped_length < sublen) {
                        btrfs_put_bbio(bbio);
                        btrfs_bio_counter_dec(fs_info);
                        return -EIO;
                }

                recover = kzalloc(sizeof(struct scrub_recover), GFP_NOFS);
                if (!recover) {
                        btrfs_put_bbio(bbio);
                        btrfs_bio_counter_dec(fs_info);
                        return -ENOMEM;
                }

                refcount_set(&recover->refs, 1);
                recover->bbio = bbio;
                recover->map_length = mapped_length;

                BUG_ON(page_index >= SCRUB_MAX_PAGES_PER_BLOCK);

                nmirrors = min(scrub_nr_raid_mirrors(bbio), BTRFS_MAX_MIRRORS);

                for (mirror_index = 0; mirror_index < nmirrors;
                     mirror_index++) {
                        struct scrub_block *sblock;
                        struct scrub_page *page;

                        sblock = sblocks_for_recheck + mirror_index;
                        sblock->sctx = sctx;

                        page = kzalloc(sizeof(*page), GFP_NOFS);
                        if (!page) {
leave_nomem:
                                spin_lock(&sctx->stat_lock);
                                sctx->stat.malloc_errors++;
                                spin_unlock(&sctx->stat_lock);
                                scrub_put_recover(fs_info, recover);
                                return -ENOMEM;
                        }
                        scrub_page_get(page);
                        sblock->pagev[page_index] = page;
                        page->sblock = sblock;
                        page->flags = flags;
                        page->generation = generation;
                        page->logical = logical;
                        page->have_csum = have_csum;
                        if (have_csum)
                                memcpy(page->csum,
                                       original_sblock->pagev[0]->csum,
                                       sctx->csum_size);

                        scrub_stripe_index_and_offset(logical,
                                                      bbio->map_type,
                                                      bbio->raid_map,
                                                      mapped_length,
                                                      bbio->num_stripes -
                                                      bbio->num_tgtdevs,
                                                      mirror_index,
                                                      &stripe_index,
                                                      &stripe_offset);
                        page->physical = bbio->stripes[stripe_index].physical +
                                         stripe_offset;
                        page->dev = bbio->stripes[stripe_index].dev;

                        BUG_ON(page_index >= original_sblock->page_count);
                        page->physical_for_dev_replace =
                                original_sblock->pagev[page_index]->
                                physical_for_dev_replace;
                        /* for missing devices, dev->bdev is NULL */
                        page->mirror_num = mirror_index + 1;
                        sblock->page_count++;
                        page->page = alloc_page(GFP_NOFS);
                        if (!page->page)
                                goto leave_nomem;

                        scrub_get_recover(recover);
                        page->recover = recover;
                }
                scrub_put_recover(fs_info, recover);
                length -= sublen;
                logical += sublen;
                page_index++;
        }

        return 0;
}

struct scrub_bio_ret {
        struct completion event;
        blk_status_t status;
};

static void scrub_bio_wait_endio(struct bio *bio)
{
        struct scrub_bio_ret *ret = bio->bi_private;

        ret->status = bio->bi_status;
        complete(&ret->event);
}

static inline int scrub_is_page_on_raid56(struct scrub_page *page)
{
        return page->recover &&
               (page->recover->bbio->map_type & BTRFS_BLOCK_GROUP_RAID56_MASK);
}

static int scrub_submit_raid56_bio_wait(struct btrfs_fs_info *fs_info,
                                        struct bio *bio,
                                        struct scrub_page *page)
{
        struct scrub_bio_ret done;
        int ret;

        init_completion(&done.event);
        done.status = 0;
        bio->bi_iter.bi_sector = page->logical >> 9;
        bio->bi_private = &done;
        bio->bi_end_io = scrub_bio_wait_endio;

        ret = raid56_parity_recover(fs_info, bio, page->recover->bbio,
                                    page->recover->map_length,
                                    page->mirror_num, 0);
        if (ret)
                return ret;

        wait_for_completion_io(&done.event);
        if (done.status)
                return -EIO;

        return 0;
}

/*
 * this function will check the on disk data for checksum errors, header
 * errors and read I/O errors. If any I/O errors happen, the exact pages
 * which are errored are marked as being bad. The goal is to enable scrub
 * to take those pages that are not errored from all the mirrors so that
 * the pages that are errored in the just handled mirror can be repaired.
 */
static void scrub_recheck_block(struct btrfs_fs_info *fs_info,
                                struct scrub_block *sblock,
                                int retry_failed_mirror)
{
        int page_num;

        sblock->no_io_error_seen = 1;

        for (page_num = 0; page_num < sblock->page_count; page_num++) {
                struct bio *bio;
                struct scrub_page *page = sblock->pagev[page_num];

                if (page->dev->bdev == NULL) {
                        page->io_error = 1;
                        sblock->no_io_error_seen = 0;
                        continue;
                }

                WARN_ON(!page->page);
                bio = btrfs_io_bio_alloc(1);
                bio_set_dev(bio, page->dev->bdev);

                bio_add_page(bio, page->page, PAGE_SIZE, 0);
                if (!retry_failed_mirror && scrub_is_page_on_raid56(page)) {
                        if (scrub_submit_raid56_bio_wait(fs_info, bio, page)) {
                                page->io_error = 1;
                                sblock->no_io_error_seen = 0;
                        }
                } else {
                        bio->bi_iter.bi_sector = page->physical >> 9;
                        bio_set_op_attrs(bio, REQ_OP_READ, 0);

                        if (btrfsic_submit_bio_wait(bio)) {
                                page->io_error = 1;
                                sblock->no_io_error_seen = 0;
                        }
                }

                bio_put(bio);
        }

        if (sblock->no_io_error_seen)
                scrub_recheck_block_checksum(sblock);
}

static inline int scrub_check_fsid(u8 fsid[],
                                   struct scrub_page *spage)
{
        struct btrfs_fs_devices *fs_devices = spage->dev->fs_devices;
        int ret;

        ret = memcmp(fsid, fs_devices->fsid, BTRFS_FSID_SIZE);
        return !ret;
}

static void scrub_recheck_block_checksum(struct scrub_block *sblock)
{
        sblock->header_error = 0;
        sblock->checksum_error = 0;
        sblock->generation_error = 0;

        if (sblock->pagev[0]->flags & BTRFS_EXTENT_FLAG_DATA)
                scrub_checksum_data(sblock);
        else
                scrub_checksum_tree_block(sblock);
}

static int scrub_repair_block_from_good_copy(struct scrub_block *sblock_bad,
                                             struct scrub_block *sblock_good)
{
        int page_num;
        int ret = 0;

        for (page_num = 0; page_num < sblock_bad->page_count; page_num++) {
                int ret_sub;

                ret_sub = scrub_repair_page_from_good_copy(sblock_bad,
                                                           sblock_good,
                                                           page_num, 1);
                if (ret_sub)
                        ret = ret_sub;
        }

        return ret;
}

static int scrub_repair_page_from_good_copy(struct scrub_block *sblock_bad,
                                            struct scrub_block *sblock_good,
                                            int page_num, int force_write)
{
        struct scrub_page *page_bad = sblock_bad->pagev[page_num];
        struct scrub_page *page_good = sblock_good->pagev[page_num];
        struct btrfs_fs_info *fs_info = sblock_bad->sctx->fs_info;

        BUG_ON(page_bad->page == NULL);
        BUG_ON(page_good->page == NULL);
        if (force_write || sblock_bad->header_error ||
            sblock_bad->checksum_error || page_bad->io_error) {
                struct bio *bio;
                int ret;

                if (!page_bad->dev->bdev) {
                        btrfs_warn_rl(fs_info,
                                "scrub_repair_page_from_good_copy(bdev == NULL) is unexpected");
                        return -EIO;
                }

                bio = btrfs_io_bio_alloc(1);
                bio_set_dev(bio, page_bad->dev->bdev);
                bio->bi_iter.bi_sector = page_bad->physical >> 9;
                bio_set_op_attrs(bio, REQ_OP_WRITE, 0);

                ret = bio_add_page(bio, page_good->page, PAGE_SIZE, 0);
                if (PAGE_SIZE != ret) {
                        bio_put(bio);
                        return -EIO;
                }

                if (btrfsic_submit_bio_wait(bio)) {
                        btrfs_dev_stat_inc_and_print(page_bad->dev,
                                BTRFS_DEV_STAT_WRITE_ERRS);
                        btrfs_dev_replace_stats_inc(
                                &fs_info->dev_replace.num_write_errors);
                        bio_put(bio);
                        return -EIO;
                }
                bio_put(bio);
        }

        return 0;
}

static void scrub_write_block_to_dev_replace(struct scrub_block *sblock)
{
        struct btrfs_fs_info *fs_info = sblock->sctx->fs_info;
        int page_num;

        /*
         * This block is used for the check of the parity on the source device,
         * so the data needn't be written into the destination device.
         */
        if (sblock->sparity)
                return;

        for (page_num = 0; page_num < sblock->page_count; page_num++) {
                int ret;

                ret = scrub_write_page_to_dev_replace(sblock, page_num);
                if (ret)
                        btrfs_dev_replace_stats_inc(
                                &fs_info->dev_replace.num_write_errors);
        }
}

static int scrub_write_page_to_dev_replace(struct scrub_block *sblock,
                                           int page_num)
{
        struct scrub_page *spage = sblock->pagev[page_num];

        BUG_ON(spage->page == NULL);
        if (spage->io_error) {
                void *mapped_buffer = kmap_atomic(spage->page);

                clear_page(mapped_buffer);
                flush_dcache_page(spage->page);
                kunmap_atomic(mapped_buffer);
        }
        return scrub_add_page_to_wr_bio(sblock->sctx, spage);
}

static int scrub_add_page_to_wr_bio(struct scrub_ctx *sctx,
                                    struct scrub_page *spage)
{
        struct scrub_bio *sbio;
        int ret;

        mutex_lock(&sctx->wr_lock);
again:
        if (!sctx->wr_curr_bio) {
                sctx->wr_curr_bio = kzalloc(sizeof(*sctx->wr_curr_bio),
                                              GFP_KERNEL);
                if (!sctx->wr_curr_bio) {
                        mutex_unlock(&sctx->wr_lock);
                        return -ENOMEM;
                }
                sctx->wr_curr_bio->sctx = sctx;
                sctx->wr_curr_bio->page_count = 0;
        }
        sbio = sctx->wr_curr_bio;
        if (sbio->page_count == 0) {
                struct bio *bio;

                sbio->physical = spage->physical_for_dev_replace;
                sbio->logical = spage->logical;
                sbio->dev = sctx->wr_tgtdev;
                bio = sbio->bio;
                if (!bio) {
                        bio = btrfs_io_bio_alloc(sctx->pages_per_wr_bio);
                        sbio->bio = bio;
                }

                bio->bi_private = sbio;
                bio->bi_end_io = scrub_wr_bio_end_io;
                bio_set_dev(bio, sbio->dev->bdev);
                bio->bi_iter.bi_sector = sbio->physical >> 9;
                bio_set_op_attrs(bio, REQ_OP_WRITE, 0);
                sbio->status = 0;
        } else if (sbio->physical + sbio->page_count * PAGE_SIZE !=
                   spage->physical_for_dev_replace ||
                   sbio->logical + sbio->page_count * PAGE_SIZE !=
                   spage->logical) {
                scrub_wr_submit(sctx);
                goto again;
        }

        ret = bio_add_page(sbio->bio, spage->page, PAGE_SIZE, 0);
        if (ret != PAGE_SIZE) {
                if (sbio->page_count < 1) {
                        bio_put(sbio->bio);
                        sbio->bio = NULL;
                        mutex_unlock(&sctx->wr_lock);
                        return -EIO;
                }
                scrub_wr_submit(sctx);
                goto again;
        }

        sbio->pagev[sbio->page_count] = spage;
        scrub_page_get(spage);
        sbio->page_count++;
        if (sbio->page_count == sctx->pages_per_wr_bio)
                scrub_wr_submit(sctx);
        mutex_unlock(&sctx->wr_lock);

        return 0;
}

static void scrub_wr_submit(struct scrub_ctx *sctx)
{
        struct scrub_bio *sbio;

        if (!sctx->wr_curr_bio)
                return;

        sbio = sctx->wr_curr_bio;
        sctx->wr_curr_bio = NULL;
        WARN_ON(!sbio->bio->bi_disk);
        scrub_pending_bio_inc(sctx);
        /* process all writes in a single worker thread. Then the block layer
         * orders the requests before sending them to the driver which
         * doubled the write performance on spinning disks when measured
         * with Linux 3.5 */
        btrfsic_submit_bio(sbio->bio);
}

static void scrub_wr_bio_end_io(struct bio *bio)
{
        struct scrub_bio *sbio = bio->bi_private;
        struct btrfs_fs_info *fs_info = sbio->dev->fs_info;

        sbio->status = bio->bi_status;
        sbio->bio = bio;

        btrfs_init_work(&sbio->work, btrfs_scrubwrc_helper,
                         scrub_wr_bio_end_io_worker, NULL, NULL);
        btrfs_queue_work(fs_info->scrub_wr_completion_workers, &sbio->work);
}

static void scrub_wr_bio_end_io_worker(struct btrfs_work *work)
{
        struct scrub_bio *sbio = container_of(work, struct scrub_bio, work);
        struct scrub_ctx *sctx = sbio->sctx;
        int i;

        WARN_ON(sbio->page_count > SCRUB_PAGES_PER_WR_BIO);
        if (sbio->status) {
                struct btrfs_dev_replace *dev_replace =
                        &sbio->sctx->fs_info->dev_replace;

                for (i = 0; i < sbio->page_count; i++) {
                        struct scrub_page *spage = sbio->pagev[i];

                        spage->io_error = 1;
                        btrfs_dev_replace_stats_inc(&dev_replace->
                                                    num_write_errors);
                }
        }

        for (i = 0; i < sbio->page_count; i++)
                scrub_page_put(sbio->pagev[i]);

        bio_put(sbio->bio);
        kfree(sbio);
        scrub_pending_bio_dec(sctx);
}

static int scrub_checksum(struct scrub_block *sblock)
{
        u64 flags;
        int ret;

        /*
         * No need to initialize these stats currently,
         * because this function only use return value
         * instead of these stats value.
         *
         * Todo:
         * always use stats
         */
        sblock->header_error = 0;
        sblock->generation_error = 0;
        sblock->checksum_error = 0;

        WARN_ON(sblock->page_count < 1);
        flags = sblock->pagev[0]->flags;
        ret = 0;
        if (flags & BTRFS_EXTENT_FLAG_DATA)
                ret = scrub_checksum_data(sblock);
        else if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK)
                ret = scrub_checksum_tree_block(sblock);
        else if (flags & BTRFS_EXTENT_FLAG_SUPER)
                (void)scrub_checksum_super(sblock);
        else
                WARN_ON(1);
        if (ret)
                scrub_handle_errored_block(sblock);

        return ret;
}

static int scrub_checksum_data(struct scrub_block *sblock)
{
        struct scrub_ctx *sctx = sblock->sctx;
        u8 csum[BTRFS_CSUM_SIZE];
        u8 *on_disk_csum;
        struct page *page;
        void *buffer;
        u32 crc = ~(u32)0;
        u64 len;
        int index;

        BUG_ON(sblock->page_count < 1);
        if (!sblock->pagev[0]->have_csum)
                return 0;

        on_disk_csum = sblock->pagev[0]->csum;
        page = sblock->pagev[0]->page;
        buffer = kmap_atomic(page);

        len = sctx->fs_info->sectorsize;
        index = 0;
        for (;;) {
                u64 l = min_t(u64, len, PAGE_SIZE);

                crc = btrfs_csum_data(buffer, crc, l);
                kunmap_atomic(buffer);
                len -= l;
                if (len == 0)
                        break;
                index++;
                BUG_ON(index >= sblock->page_count);
                BUG_ON(!sblock->pagev[index]->page);
                page = sblock->pagev[index]->page;
                buffer = kmap_atomic(page);
        }

        btrfs_csum_final(crc, csum);
        if (memcmp(csum, on_disk_csum, sctx->csum_size))
                sblock->checksum_error = 1;

        return sblock->checksum_error;
}

static int scrub_checksum_tree_block(struct scrub_block *sblock)
{
        struct scrub_ctx *sctx = sblock->sctx;
        struct btrfs_header *h;
        struct btrfs_fs_info *fs_info = sctx->fs_info;
        u8 calculated_csum[BTRFS_CSUM_SIZE];
        u8 on_disk_csum[BTRFS_CSUM_SIZE];
        struct page *page;
        void *mapped_buffer;
        u64 mapped_size;
        void *p;
        u32 crc = ~(u32)0;
        u64 len;
        int index;

        BUG_ON(sblock->page_count < 1);
        page = sblock->pagev[0]->page;
        mapped_buffer = kmap_atomic(page);
        h = (struct btrfs_header *)mapped_buffer;
        memcpy(on_disk_csum, h->csum, sctx->csum_size);

        /*
         * we don't use the getter functions here, as we
         * a) don't have an extent buffer and
         * b) the page is already kmapped
         */
        if (sblock->pagev[0]->logical != btrfs_stack_header_bytenr(h))
                sblock->header_error = 1;

        if (sblock->pagev[0]->generation != btrfs_stack_header_generation(h)) {
                sblock->header_error = 1;
                sblock->generation_error = 1;
        }

        if (!scrub_check_fsid(h->fsid, sblock->pagev[0]))
                sblock->header_error = 1;

        if (memcmp(h->chunk_tree_uuid, fs_info->chunk_tree_uuid,
                   BTRFS_UUID_SIZE))
                sblock->header_error = 1;

        len = sctx->fs_info->nodesize - BTRFS_CSUM_SIZE;
        mapped_size = PAGE_SIZE - BTRFS_CSUM_SIZE;
        p = ((u8 *)mapped_buffer) + BTRFS_CSUM_SIZE;
        index = 0;
        for (;;) {
                u64 l = min_t(u64, len, mapped_size);

                crc = btrfs_csum_data(p, crc, l);
                kunmap_atomic(mapped_buffer);
                len -= l;
                if (len == 0)
                        break;
                index++;
                BUG_ON(index >= sblock->page_count);
                BUG_ON(!sblock->pagev[index]->page);
                page = sblock->pagev[index]->page;
                mapped_buffer = kmap_atomic(page);
                mapped_size = PAGE_SIZE;
                p = mapped_buffer;
        }

        btrfs_csum_final(crc, calculated_csum);
        if (memcmp(calculated_csum, on_disk_csum, sctx->csum_size))
                sblock->checksum_error = 1;

        return sblock->header_error || sblock->checksum_error;
}

static int scrub_checksum_super(struct scrub_block *sblock)
{
        struct btrfs_super_block *s;
        struct scrub_ctx *sctx = sblock->sctx;
        u8 calculated_csum[BTRFS_CSUM_SIZE];
        u8 on_disk_csum[BTRFS_CSUM_SIZE];
        struct page *page;
        void *mapped_buffer;
        u64 mapped_size;
        void *p;
        u32 crc = ~(u32)0;
        int fail_gen = 0;
        int fail_cor = 0;
        u64 len;
        int index;

        BUG_ON(sblock->page_count < 1);
        page = sblock->pagev[0]->page;
        mapped_buffer = kmap_atomic(page);
        s = (struct btrfs_super_block *)mapped_buffer;
        memcpy(on_disk_csum, s->csum, sctx->csum_size);

        if (sblock->pagev[0]->logical != btrfs_super_bytenr(s))
                ++fail_cor;

        if (sblock->pagev[0]->generation != btrfs_super_generation(s))
                ++fail_gen;

        if (!scrub_check_fsid(s->fsid, sblock->pagev[0]))
                ++fail_cor;

        len = BTRFS_SUPER_INFO_SIZE - BTRFS_CSUM_SIZE;
        mapped_size = PAGE_SIZE - BTRFS_CSUM_SIZE;
        p = ((u8 *)mapped_buffer) + BTRFS_CSUM_SIZE;
        index = 0;
        for (;;) {
                u64 l = min_t(u64, len, mapped_size);

                crc = btrfs_csum_data(p, crc, l);
                kunmap_atomic(mapped_buffer);
                len -= l;
                if (len == 0)
                        break;
                index++;
                BUG_ON(index >= sblock->page_count);
                BUG_ON(!sblock->pagev[index]->page);
                page = sblock->pagev[index]->page;
                mapped_buffer = kmap_atomic(page);
                mapped_size = PAGE_SIZE;
                p = mapped_buffer;
        }

        btrfs_csum_final(crc, calculated_csum);
        if (memcmp(calculated_csum, on_disk_csum, sctx->csum_size))
                ++fail_cor;

        if (fail_cor + fail_gen) {
                /*
                 * if we find an error in a super block, we just report it.
                 * They will get written with the next transaction commit
                 * anyway
                 */
                spin_lock(&sctx->stat_lock);
                ++sctx->stat.super_errors;
                spin_unlock(&sctx->stat_lock);
                if (fail_cor)
                        btrfs_dev_stat_inc_and_print(sblock->pagev[0]->dev,
                                BTRFS_DEV_STAT_CORRUPTION_ERRS);
                else
                        btrfs_dev_stat_inc_and_print(sblock->pagev[0]->dev,
                                BTRFS_DEV_STAT_GENERATION_ERRS);
        }

        return fail_cor + fail_gen;
}

static void scrub_block_get(struct scrub_block *sblock)
{
        refcount_inc(&sblock->refs);
}

static void scrub_block_put(struct scrub_block *sblock)
{
        if (refcount_dec_and_test(&sblock->refs)) {
                int i;

                if (sblock->sparity)
                        scrub_parity_put(sblock->sparity);

                for (i = 0; i < sblock->page_count; i++)
                        scrub_page_put(sblock->pagev[i]);
                kfree(sblock);
        }
}

static void scrub_page_get(struct scrub_page *spage)
{
        atomic_inc(&spage->refs);
}

static void scrub_page_put(struct scrub_page *spage)
{
        if (atomic_dec_and_test(&spage->refs)) {
                if (spage->page)
                        __free_page(spage->page);
                kfree(spage);
        }
}

static void scrub_submit(struct scrub_ctx *sctx)
{
        struct scrub_bio *sbio;

        if (sctx->curr == -1)
                return;

        sbio = sctx->bios[sctx->curr];
        sctx->curr = -1;
        scrub_pending_bio_inc(sctx);
        btrfsic_submit_bio(sbio->bio);
}

static int scrub_add_page_to_rd_bio(struct scrub_ctx *sctx,
                                    struct scrub_page *spage)
{
        struct scrub_block *sblock = spage->sblock;
        struct scrub_bio *sbio;
        int ret;

again:
        /*
         * grab a fresh bio or wait for one to become available
         */
        while (sctx->curr == -1) {
                spin_lock(&sctx->list_lock);
                sctx->curr = sctx->first_free;
                if (sctx->curr != -1) {
                        sctx->first_free = sctx->bios[sctx->curr]->next_free;
                        sctx->bios[sctx->curr]->next_free = -1;
                        sctx->bios[sctx->curr]->page_count = 0;
                        spin_unlock(&sctx->list_lock);
                } else {
                        spin_unlock(&sctx->list_lock);
                        wait_event(sctx->list_wait, sctx->first_free != -1);
                }
        }
        sbio = sctx->bios[sctx->curr];
        if (sbio->page_count == 0) {
                struct bio *bio;

                sbio->physical = spage->physical;
                sbio->logical = spage->logical;
                sbio->dev = spage->dev;
                bio = sbio->bio;
                if (!bio) {
                        bio = btrfs_io_bio_alloc(sctx->pages_per_rd_bio);
                        sbio->bio = bio;
                }

                bio->bi_private = sbio;
                bio->bi_end_io = scrub_bio_end_io;
                bio_set_dev(bio, sbio->dev->bdev);
                bio->bi_iter.bi_sector = sbio->physical >> 9;
                bio_set_op_attrs(bio, REQ_OP_READ, 0);
                sbio->status = 0;
        } else if (sbio->physical + sbio->page_count * PAGE_SIZE !=
                   spage->physical ||
                   sbio->logical + sbio->page_count * PAGE_SIZE !=
                   spage->logical ||
                   sbio->dev != spage->dev) {
                scrub_submit(sctx);
                goto again;
        }

        sbio->pagev[sbio->page_count] = spage;
        ret = bio_add_page(sbio->bio, spage->page, PAGE_SIZE, 0);
        if (ret != PAGE_SIZE) {
                if (sbio->page_count < 1) {
                        bio_put(sbio->bio);
                        sbio->bio = NULL;
                        return -EIO;
                }
                scrub_submit(sctx);
                goto again;
        }

        scrub_block_get(sblock); /* one for the page added to the bio */
        atomic_inc(&sblock->outstanding_pages);
        sbio->page_count++;
        if (sbio->page_count == sctx->pages_per_rd_bio)
                scrub_submit(sctx);

        return 0;
}

static void scrub_missing_raid56_end_io(struct bio *bio)
{
        struct scrub_block *sblock = bio->bi_private;
        struct btrfs_fs_info *fs_info = sblock->sctx->fs_info;

        if (bio->bi_status)
                sblock->no_io_error_seen = 0;

        bio_put(bio);

        btrfs_queue_work(fs_info->scrub_workers, &sblock->work);
}

static void scrub_missing_raid56_worker(struct btrfs_work *work)
{
        struct scrub_block *sblock = container_of(work, struct scrub_block, work);
        struct scrub_ctx *sctx = sblock->sctx;
        struct btrfs_fs_info *fs_info = sctx->fs_info;
        u64 logical;
        struct btrfs_device *dev;

        logical = sblock->pagev[0]->logical;
        dev = sblock->pagev[0]->dev;

        if (sblock->no_io_error_seen)
                scrub_recheck_block_checksum(sblock);

        if (!sblock->no_io_error_seen) {
                spin_lock(&sctx->stat_lock);
                sctx->stat.read_errors++;
                spin_unlock(&sctx->stat_lock);
                btrfs_err_rl_in_rcu(fs_info,
                        "IO error rebuilding logical %llu for dev %s",
                        logical, rcu_str_deref(dev->name));
        } else if (sblock->header_error || sblock->checksum_error) {
                spin_lock(&sctx->stat_lock);
                sctx->stat.uncorrectable_errors++;
                spin_unlock(&sctx->stat_lock);
                btrfs_err_rl_in_rcu(fs_info,
                        "failed to rebuild valid logical %llu for dev %s",
                        logical, rcu_str_deref(dev->name));
        } else {
                scrub_write_block_to_dev_replace(sblock);
        }

        scrub_block_put(sblock);

        if (sctx->is_dev_replace && sctx->flush_all_writes) {
                mutex_lock(&sctx->wr_lock);
                scrub_wr_submit(sctx);
                mutex_unlock(&sctx->wr_lock);
        }

        scrub_pending_bio_dec(sctx);
}

static void scrub_missing_raid56_pages(struct scrub_block *sblock)
{
        struct scrub_ctx *sctx = sblock->sctx;
        struct btrfs_fs_info *fs_info = sctx->fs_info;
        u64 length = sblock->page_count * PAGE_SIZE;
        u64 logical = sblock->pagev[0]->logical;
        struct btrfs_bio *bbio = NULL;
        struct bio *bio;
        struct btrfs_raid_bio *rbio;
        int ret;
        int i;

        btrfs_bio_counter_inc_blocked(fs_info);
        ret = btrfs_map_sblock(fs_info, BTRFS_MAP_GET_READ_MIRRORS, logical,
                        &length, &bbio);
        if (ret || !bbio || !bbio->raid_map)
                goto bbio_out;

        if (WARN_ON(!sctx->is_dev_replace ||
                    !(bbio->map_type & BTRFS_BLOCK_GROUP_RAID56_MASK))) {
                /*
                 * We shouldn't be scrubbing a missing device. Even for dev
                 * replace, we should only get here for RAID 5/6. We either
                 * managed to mount something with no mirrors remaining or
                 * there's a bug in scrub_remap_extent()/btrfs_map_block().
                 */
                goto bbio_out;
        }

        bio = btrfs_io_bio_alloc(0);
        bio->bi_iter.bi_sector = logical >> 9;
        bio->bi_private = sblock;
        bio->bi_end_io = scrub_missing_raid56_end_io;

        rbio = raid56_alloc_missing_rbio(fs_info, bio, bbio, length);
        if (!rbio)
                goto rbio_out;

        for (i = 0; i < sblock->page_count; i++) {
                struct scrub_page *spage = sblock->pagev[i];

                raid56_add_scrub_pages(rbio, spage->page, spage->logical);
        }

        btrfs_init_work(&sblock->work, btrfs_scrub_helper,
                        scrub_missing_raid56_worker, NULL, NULL);
        scrub_block_get(sblock);
        scrub_pending_bio_inc(sctx);
        raid56_submit_missing_rbio(rbio);
        return;

rbio_out:
        bio_put(bio);
bbio_out:
        btrfs_bio_counter_dec(fs_info);
        btrfs_put_bbio(bbio);
        spin_lock(&sctx->stat_lock);
        sctx->stat.malloc_errors++;
        spin_unlock(&sctx->stat_lock);
}

static int scrub_pages(struct scrub_ctx *sctx, u64 logical, u64 len,
                       u64 physical, struct btrfs_device *dev, u64 flags,
                       u64 gen, int mirror_num, u8 *csum, int force,
                       u64 physical_for_dev_replace)
{
        struct scrub_block *sblock;
        int index;

        sblock = kzalloc(sizeof(*sblock), GFP_KERNEL);
        if (!sblock) {
                spin_lock(&sctx->stat_lock);
                sctx->stat.malloc_errors++;
                spin_unlock(&sctx->stat_lock);
                return -ENOMEM;
        }

        /* one ref inside this function, plus one for each page added to
         * a bio later on */
        refcount_set(&sblock->refs, 1);
        sblock->sctx = sctx;
        sblock->no_io_error_seen = 1;

        for (index = 0; len > 0; index++) {
                struct scrub_page *spage;
                u64 l = min_t(u64, len, PAGE_SIZE);

                spage = kzalloc(sizeof(*spage), GFP_KERNEL);
                if (!spage) {
leave_nomem:
                        spin_lock(&sctx->stat_lock);
                        sctx->stat.malloc_errors++;
                        spin_unlock(&sctx->stat_lock);
                        scrub_block_put(sblock);
                        return -ENOMEM;
                }
                BUG_ON(index >= SCRUB_MAX_PAGES_PER_BLOCK);
                scrub_page_get(spage);
                sblock->pagev[index] = spage;
                spage->sblock = sblock;
                spage->dev = dev;
                spage->flags = flags;
                spage->generation = gen;
                spage->logical = logical;
                spage->physical = physical;
                spage->physical_for_dev_replace = physical_for_dev_replace;
                spage->mirror_num = mirror_num;
                if (csum) {
                        spage->have_csum = 1;
                        memcpy(spage->csum, csum, sctx->csum_size);
                } else {
                        spage->have_csum = 0;
                }
                sblock->page_count++;
                spage->page = alloc_page(GFP_KERNEL);
                if (!spage->page)
                        goto leave_nomem;
                len -= l;
                logical += l;
                physical += l;
                physical_for_dev_replace += l;
        }

        WARN_ON(sblock->page_count == 0);
        if (dev->missing) {
                /*
                 * This case should only be hit for RAID 5/6 device replace. See
                 * the comment in scrub_missing_raid56_pages() for details.
                 */
                scrub_missing_raid56_pages(sblock);
        } else {
                for (index = 0; index < sblock->page_count; index++) {
                        struct scrub_page *spage = sblock->pagev[index];
                        int ret;

                        ret = scrub_add_page_to_rd_bio(sctx, spage);
                        if (ret) {
                                scrub_block_put(sblock);
                                return ret;
                        }
                }

                if (force)
                        scrub_submit(sctx);
        }

        /* last one frees, either here or in bio completion for last page */
        scrub_block_put(sblock);
        return 0;
}

static void scrub_bio_end_io(struct bio *bio)
{
        struct scrub_bio *sbio = bio->bi_private;
        struct btrfs_fs_info *fs_info = sbio->dev->fs_info;

        sbio->status = bio->bi_status;
        sbio->bio = bio;

        btrfs_queue_work(fs_info->scrub_workers, &sbio->work);
}

static void scrub_bio_end_io_worker(struct btrfs_work *work)
{
        struct scrub_bio *sbio = container_of(work, struct scrub_bio, work);
        struct scrub_ctx *sctx = sbio->sctx;
        int i;

        BUG_ON(sbio->page_count > SCRUB_PAGES_PER_RD_BIO);
        if (sbio->status) {
                for (i = 0; i < sbio->page_count; i++) {
                        struct scrub_page *spage = sbio->pagev[i];

                        spage->io_error = 1;
                        spage->sblock->no_io_error_seen = 0;
                }
        }

        /* now complete the scrub_block items that have all pages completed */
        for (i = 0; i < sbio->page_count; i++) {
                struct scrub_page *spage = sbio->pagev[i];
                struct scrub_block *sblock = spage->sblock;

                if (atomic_dec_and_test(&sblock->outstanding_pages))
                        scrub_block_complete(sblock);
                scrub_block_put(sblock);
        }

        bio_put(sbio->bio);
        sbio->bio = NULL;
        spin_lock(&sctx->list_lock);
        sbio->next_free = sctx->first_free;
        sctx->first_free = sbio->index;
        spin_unlock(&sctx->list_lock);

        if (sctx->is_dev_replace && sctx->flush_all_writes) {
                mutex_lock(&sctx->wr_lock);
                scrub_wr_submit(sctx);
                mutex_unlock(&sctx->wr_lock);
        }

        scrub_pending_bio_dec(sctx);
}

static inline void __scrub_mark_bitmap(struct scrub_parity *sparity,
                                       unsigned long *bitmap,
                                       u64 start, u64 len)
{
        u64 offset;
        u64 nsectors64;
        u32 nsectors;
        int sectorsize = sparity->sctx->fs_info->sectorsize;

        if (len >= sparity->stripe_len) {
                bitmap_set(bitmap, 0, sparity->nsectors);
                return;
        }

        start -= sparity->logic_start;
        start = div64_u64_rem(start, sparity->stripe_len, &offset);
        offset = div_u64(offset, sectorsize);
        nsectors64 = div_u64(len, sectorsize);

        ASSERT(nsectors64 < UINT_MAX);
        nsectors = (u32)nsectors64;

        if (offset + nsectors <= sparity->nsectors) {
                bitmap_set(bitmap, offset, nsectors);
                return;
        }

        bitmap_set(bitmap, offset, sparity->nsectors - offset);
        bitmap_set(bitmap, 0, nsectors - (sparity->nsectors - offset));
}

static inline void scrub_parity_mark_sectors_error(struct scrub_parity *sparity,
                                                   u64 start, u64 len)
{
        __scrub_mark_bitmap(sparity, sparity->ebitmap, start, len);
}

static inline void scrub_parity_mark_sectors_data(struct scrub_parity *sparity,
                                                  u64 start, u64 len)
{
        __scrub_mark_bitmap(sparity, sparity->dbitmap, start, len);
}

static void scrub_block_complete(struct scrub_block *sblock)
{
        int corrupted = 0;

        if (!sblock->no_io_error_seen) {
                corrupted = 1;
                scrub_handle_errored_block(sblock);
        } else {
                /*
                 * if has checksum error, write via repair mechanism in
                 * dev replace case, otherwise write here in dev replace
                 * case.
                 */
                corrupted = scrub_checksum(sblock);
                if (!corrupted && sblock->sctx->is_dev_replace)
                        scrub_write_block_to_dev_replace(sblock);
        }

        if (sblock->sparity && corrupted && !sblock->data_corrected) {
                u64 start = sblock->pagev[0]->logical;
                u64 end = sblock->pagev[sblock->page_count - 1]->logical +
                          PAGE_SIZE;

                scrub_parity_mark_sectors_error(sblock->sparity,
                                                start, end - start);
        }
}

static int scrub_find_csum(struct scrub_ctx *sctx, u64 logical, u8 *csum)
{
        struct btrfs_ordered_sum *sum = NULL;
        unsigned long index;
        unsigned long num_sectors;

        while (!list_empty(&sctx->csum_list)) {
                sum = list_first_entry(&sctx->csum_list,
                                       struct btrfs_ordered_sum, list);
                if (sum->bytenr > logical)
                        return 0;
                if (sum->bytenr + sum->len > logical)
                        break;

                ++sctx->stat.csum_discards;
                list_del(&sum->list);
                kfree(sum);
                sum = NULL;
        }
        if (!sum)
                return 0;

        index = div_u64(logical - sum->bytenr, sctx->fs_info->sectorsize);
        ASSERT(index < UINT_MAX);

        num_sectors = sum->len / sctx->fs_info->sectorsize;
        memcpy(csum, sum->sums + index, sctx->csum_size);
        if (index == num_sectors - 1) {
                list_del(&sum->list);
                kfree(sum);
        }
        return 1;
}

/* scrub extent tries to collect up to 64 kB for each bio */
static int scrub_extent(struct scrub_ctx *sctx, u64 logical, u64 len,
                        u64 physical, struct btrfs_device *dev, u64 flags,
                        u64 gen, int mirror_num, u64 physical_for_dev_replace)
{
        int ret;
        u8 csum[BTRFS_CSUM_SIZE];
        u32 blocksize;

        if (flags & BTRFS_EXTENT_FLAG_DATA) {
                blocksize = sctx->fs_info->sectorsize;
                spin_lock(&sctx->stat_lock);
                sctx->stat.data_extents_scrubbed++;
                sctx->stat.data_bytes_scrubbed += len;
                spin_unlock(&sctx->stat_lock);
        } else if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK) {
                blocksize = sctx->fs_info->nodesize;
                spin_lock(&sctx->stat_lock);
                sctx->stat.tree_extents_scrubbed++;
                sctx->stat.tree_bytes_scrubbed += len;
                spin_unlock(&sctx->stat_lock);
        } else {
                blocksize = sctx->fs_info->sectorsize;
                WARN_ON(1);
        }

        while (len) {
                u64 l = min_t(u64, len, blocksize);
                int have_csum = 0;

                if (flags & BTRFS_EXTENT_FLAG_DATA) {
                        /* push csums to sbio */
                        have_csum = scrub_find_csum(sctx, logical, csum);
                        if (have_csum == 0)
                                ++sctx->stat.no_csum;
                        if (sctx->is_dev_replace && !have_csum) {
                                ret = copy_nocow_pages(sctx, logical, l,
                                                       mirror_num,
                                                      physical_for_dev_replace);
                                goto behind_scrub_pages;
                        }
                }
                ret = scrub_pages(sctx, logical, l, physical, dev, flags, gen,
                                  mirror_num, have_csum ? csum : NULL, 0,
                                  physical_for_dev_replace);
behind_scrub_pages:
                if (ret)
                        return ret;
                len -= l;
                logical += l;
                physical += l;
                physical_for_dev_replace += l;
        }
        return 0;
}

static int scrub_pages_for_parity(struct scrub_parity *sparity,
                                  u64 logical, u64 len,
                                  u64 physical, struct btrfs_device *dev,
                                  u64 flags, u64 gen, int mirror_num, u8 *csum)
{
        struct scrub_ctx *sctx = sparity->sctx;
        struct scrub_block *sblock;
        int index;

        sblock = kzalloc(sizeof(*sblock), GFP_KERNEL);
        if (!sblock) {
                spin_lock(&sctx->stat_lock);
                sctx->stat.malloc_errors++;
                spin_unlock(&sctx->stat_lock);
                return -ENOMEM;
        }

        /* one ref inside this function, plus one for each page added to
         * a bio later on */
        refcount_set(&sblock->refs, 1);
        sblock->sctx = sctx;
        sblock->no_io_error_seen = 1;
        sblock->sparity = sparity;
        scrub_parity_get(sparity);

        for (index = 0; len > 0; index++) {
                struct scrub_page *spage;
                u64 l = min_t(u64, len, PAGE_SIZE);

                spage = kzalloc(sizeof(*spage), GFP_KERNEL);
                if (!spage) {
leave_nomem:
                        spin_lock(&sctx->stat_lock);
                        sctx->stat.malloc_errors++;
                        spin_unlock(&sctx->stat_lock);
                        scrub_block_put(sblock);
                        return -ENOMEM;
                }
                BUG_ON(index >= SCRUB_MAX_PAGES_PER_BLOCK);
                /* For scrub block */
                scrub_page_get(spage);
                sblock->pagev[index] = spage;
                /* For scrub parity */
                scrub_page_get(spage);
                list_add_tail(&spage->list, &sparity->spages);
                spage->sblock = sblock;
                spage->dev = dev;
                spage->flags = flags;
                spage->generation = gen;
                spage->logical = logical;
                spage->physical = physical;
                spage->mirror_num = mirror_num;
                if (csum) {
                        spage->have_csum = 1;
                        memcpy(spage->csum, csum, sctx->csum_size);
                } else {
                        spage->have_csum = 0;
                }
                sblock->page_count++;
                spage->page = alloc_page(GFP_KERNEL);
                if (!spage->page)
                        goto leave_nomem;
                len -= l;
                logical += l;
                physical += l;
        }

        WARN_ON(sblock->page_count == 0);
        for (index = 0; index < sblock->page_count; index++) {
                struct scrub_page *spage = sblock->pagev[index];
                int ret;

                ret = scrub_add_page_to_rd_bio(sctx, spage);
                if (ret) {
                        scrub_block_put(sblock);
                        return ret;
                }
        }

        /* last one frees, either here or in bio completion for last page */
        scrub_block_put(sblock);
        return 0;
}

static int scrub_extent_for_parity(struct scrub_parity *sparity,
                                   u64 logical, u64 len,
                                   u64 physical, struct btrfs_device *dev,
                                   u64 flags, u64 gen, int mirror_num)
{
        struct scrub_ctx *sctx = sparity->sctx;
        int ret;
        u8 csum[BTRFS_CSUM_SIZE];
        u32 blocksize;

        if (dev->missing) {
                scrub_parity_mark_sectors_error(sparity, logical, len);
                return 0;
        }

        if (flags & BTRFS_EXTENT_FLAG_DATA) {
                blocksize = sctx->fs_info->sectorsize;
        } else if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK) {
                blocksize = sctx->fs_info->nodesize;
        } else {
                blocksize = sctx->fs_info->sectorsize;
                WARN_ON(1);
        }

        while (len) {
                u64 l = min_t(u64, len, blocksize);
                int have_csum = 0;

                if (flags & BTRFS_EXTENT_FLAG_DATA) {
                        /* push csums to sbio */
                        have_csum = scrub_find_csum(sctx, logical, csum);
                        if (have_csum == 0)
                                goto skip;
                }
                ret = scrub_pages_for_parity(sparity, logical, l, physical, dev,
                                             flags, gen, mirror_num,
                                             have_csum ? csum : NULL);
                if (ret)
                        return ret;
skip:
                len -= l;
                logical += l;
                physical += l;
        }
        return 0;
}

/*
 * Given a physical address, this will calculate it's
 * logical offset. if this is a parity stripe, it will return
 * the most left data stripe's logical offset.
 *
 * return 0 if it is a data stripe, 1 means parity stripe.
 */
static int get_raid56_logic_offset(u64 physical, int num,
                                   struct map_lookup *map, u64 *offset,
                                   u64 *stripe_start)
{
        int i;
        int j = 0;
        u64 stripe_nr;
        u64 last_offset;
        u32 stripe_index;
        u32 rot;

        last_offset = (physical - map->stripes[num].physical) *
                      nr_data_stripes(map);
        if (stripe_start)
                *stripe_start = last_offset;

        *offset = last_offset;
        for (i = 0; i < nr_data_stripes(map); i++) {
                *offset = last_offset + i * map->stripe_len;

                stripe_nr = div64_u64(*offset, map->stripe_len);
                stripe_nr = div_u64(stripe_nr, nr_data_stripes(map));

                /* Work out the disk rotation on this stripe-set */
                stripe_nr = div_u64_rem(stripe_nr, map->num_stripes, &rot);
                /* calculate which stripe this data locates */
                rot += i;
                stripe_index = rot % map->num_stripes;
                if (stripe_index == num)
                        return 0;
                if (stripe_index < num)
                        j++;
        }
        *offset = last_offset + j * map->stripe_len;
        return 1;
}

static void scrub_free_parity(struct scrub_parity *sparity)
{
        struct scrub_ctx *sctx = sparity->sctx;
        struct scrub_page *curr, *next;
        int nbits;

        nbits = bitmap_weight(sparity->ebitmap, sparity->nsectors);
        if (nbits) {
                spin_lock(&sctx->stat_lock);
                sctx->stat.read_errors += nbits;
                sctx->stat.uncorrectable_errors += nbits;
                spin_unlock(&sctx->stat_lock);
        }

        list_for_each_entry_safe(curr, next, &sparity->spages, list) {
                list_del_init(&curr->list);
                scrub_page_put(curr);
        }

        kfree(sparity);
}

static void scrub_parity_bio_endio_worker(struct btrfs_work *work)
{
        struct scrub_parity *sparity = container_of(work, struct scrub_parity,
                                                    work);
        struct scrub_ctx *sctx = sparity->sctx;

        scrub_free_parity(sparity);
        scrub_pending_bio_dec(sctx);
}

static void scrub_parity_bio_endio(struct bio *bio)
{
        struct scrub_parity *sparity = (struct scrub_parity *)bio->bi_private;
        struct btrfs_fs_info *fs_info = sparity->sctx->fs_info;

        if (bio->bi_status)
                bitmap_or(sparity->ebitmap, sparity->ebitmap, sparity->dbitmap,
                          sparity->nsectors);

        bio_put(bio);

        btrfs_init_work(&sparity->work, btrfs_scrubparity_helper,
                        scrub_parity_bio_endio_worker, NULL, NULL);
        btrfs_queue_work(fs_info->scrub_parity_workers, &sparity->work);
}

static void scrub_parity_check_and_repair(struct scrub_parity *sparity)
{
        struct scrub_ctx *sctx = sparity->sctx;
        struct btrfs_fs_info *fs_info = sctx->fs_info;
        struct bio *bio;
        struct btrfs_raid_bio *rbio;
        struct btrfs_bio *bbio = NULL;
        u64 length;
        int ret;

        if (!bitmap_andnot(sparity->dbitmap, sparity->dbitmap, sparity->ebitmap,
                           sparity->nsectors))
                goto out;

        length = sparity->logic_end - sparity->logic_start;

        btrfs_bio_counter_inc_blocked(fs_info);
        ret = btrfs_map_sblock(fs_info, BTRFS_MAP_WRITE, sparity->logic_start,
                               &length, &bbio);
        if (ret || !bbio || !bbio->raid_map)
                goto bbio_out;

        bio = btrfs_io_bio_alloc(0);
        bio->bi_iter.bi_sector = sparity->logic_start >> 9;
        bio->bi_private = sparity;
        bio->bi_end_io = scrub_parity_bio_endio;

        rbio = raid56_parity_alloc_scrub_rbio(fs_info, bio, bbio,
                                              length, sparity->scrub_dev,
                                              sparity->dbitmap,
                                              sparity->nsectors);
        if (!rbio)
                goto rbio_out;

        scrub_pending_bio_inc(sctx);
        raid56_parity_submit_scrub_rbio(rbio);
        return;

rbio_out:
        bio_put(bio);
bbio_out:
        btrfs_bio_counter_dec(fs_info);
        btrfs_put_bbio(bbio);
        bitmap_or(sparity->ebitmap, sparity->ebitmap, sparity->dbitmap,
                  sparity->nsectors);
        spin_lock(&sctx->stat_lock);
        sctx->stat.malloc_errors++;
        spin_unlock(&sctx->stat_lock);
out:
        scrub_free_parity(sparity);
}

static inline int scrub_calc_parity_bitmap_len(int nsectors)
{
        return DIV_ROUND_UP(nsectors, BITS_PER_LONG) * sizeof(long);
}

static void scrub_parity_get(struct scrub_parity *sparity)
{
        refcount_inc(&sparity->refs);
}

static void scrub_parity_put(struct scrub_parity *sparity)
{
        if (!refcount_dec_and_test(&sparity->refs))
                return;

        scrub_parity_check_and_repair(sparity);
}

static noinline_for_stack int scrub_raid56_parity(struct scrub_ctx *sctx,
                                                  struct map_lookup *map,
                                                  struct btrfs_device *sdev,
                                                  struct btrfs_path *path,
                                                  u64 logic_start,
                                                  u64 logic_end)
{
        struct btrfs_fs_info *fs_info = sctx->fs_info;
        struct btrfs_root *root = fs_info->extent_root;
        struct btrfs_root *csum_root = fs_info->csum_root;
        struct btrfs_extent_item *extent;
        struct btrfs_bio *bbio = NULL;
        u64 flags;
        int ret;
        int slot;
        struct extent_buffer *l;
        struct btrfs_key key;
        u64 generation;
        u64 extent_logical;
        u64 extent_physical;
        u64 extent_len;
        u64 mapped_length;
        struct btrfs_device *extent_dev;
        struct scrub_parity *sparity;
        int nsectors;
        int bitmap_len;
        int extent_mirror_num;
        int stop_loop = 0;

        nsectors = div_u64(map->stripe_len, fs_info->sectorsize);
        bitmap_len = scrub_calc_parity_bitmap_len(nsectors);
        sparity = kzalloc(sizeof(struct scrub_parity) + 2 * bitmap_len,
                          GFP_NOFS);
        if (!sparity) {
                spin_lock(&sctx->stat_lock);
                sctx->stat.malloc_errors++;
                spin_unlock(&sctx->stat_lock);
                return -ENOMEM;
        }

        sparity->stripe_len = map->stripe_len;
        sparity->nsectors = nsectors;
        sparity->sctx = sctx;
        sparity->scrub_dev = sdev;
        sparity->logic_start = logic_start;
        sparity->logic_end = logic_end;
        refcount_set(&sparity->refs, 1);
        INIT_LIST_HEAD(&sparity->spages);
        sparity->dbitmap = sparity->bitmap;
        sparity->ebitmap = (void *)sparity->bitmap + bitmap_len;

        ret = 0;
        while (logic_start < logic_end) {
                if (btrfs_fs_incompat(fs_info, SKINNY_METADATA))
                        key.type = BTRFS_METADATA_ITEM_KEY;
                else
                        key.type = BTRFS_EXTENT_ITEM_KEY;
                key.objectid = logic_start;
                key.offset = (u64)-1;

                ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
                if (ret < 0)
                        goto out;

                if (ret > 0) {
                        ret = btrfs_previous_extent_item(root, path, 0);
                        if (ret < 0)
                                goto out;
                        if (ret > 0) {
                                btrfs_release_path(path);
                                ret = btrfs_search_slot(NULL, root, &key,
                                                        path, 0, 0);
                                if (ret < 0)
                                        goto out;
                        }
                }

                stop_loop = 0;
                while (1) {
                        u64 bytes;

                        l = path->nodes[0];
                        slot = path->slots[0];
                        if (slot >= btrfs_header_nritems(l)) {
                                ret = btrfs_next_leaf(root, path);
                                if (ret == 0)
                                        continue;
                                if (ret < 0)
                                        goto out;

                                stop_loop = 1;
                                break;
                        }
                        btrfs_item_key_to_cpu(l, &key, slot);

                        if (key.type != BTRFS_EXTENT_ITEM_KEY &&
                            key.type != BTRFS_METADATA_ITEM_KEY)
                                goto next;

                        if (key.type == BTRFS_METADATA_ITEM_KEY)
                                bytes = fs_info->nodesize;
                        else
                                bytes = key.offset;

                        if (key.objectid + bytes <= logic_start)
                                goto next;

                        if (key.objectid >= logic_end) {
                                stop_loop = 1;
                                break;
                        }

                        while (key.objectid >= logic_start + map->stripe_len)
                                logic_start += map->stripe_len;

                        extent = btrfs_item_ptr(l, slot,
                                                struct btrfs_extent_item);
                        flags = btrfs_extent_flags(l, extent);
                        generation = btrfs_extent_generation(l, extent);

                        if ((flags & BTRFS_EXTENT_FLAG_TREE_BLOCK) &&
                            (key.objectid < logic_start ||
                             key.objectid + bytes >
                             logic_start + map->stripe_len)) {
                                btrfs_err(fs_info,
                                          "scrub: tree block %llu spanning stripes, ignored. logical=%llu",
                                          key.objectid, logic_start);
                                spin_lock(&sctx->stat_lock);
                                sctx->stat.uncorrectable_errors++;
                                spin_unlock(&sctx->stat_lock);
                                goto next;
                        }
again:
                        extent_logical = key.objectid;
                        extent_len = bytes;

                        if (extent_logical < logic_start) {
                                extent_len -= logic_start - extent_logical;
                                extent_logical = logic_start;
                        }

                        if (extent_logical + extent_len >
                            logic_start + map->stripe_len)
                                extent_len = logic_start + map->stripe_len -
                                             extent_logical;

                        scrub_parity_mark_sectors_data(sparity, extent_logical,
                                                       extent_len);

                        mapped_length = extent_len;
                        bbio = NULL;
                        ret = btrfs_map_block(fs_info, BTRFS_MAP_READ,
                                        extent_logical, &mapped_length, &bbio,
                                        0);
                        if (!ret) {
                                if (!bbio || mapped_length < extent_len)
                                        ret = -EIO;
                        }
                        if (ret) {
                                btrfs_put_bbio(bbio);
                                goto out;
                        }
                        extent_physical = bbio->stripes[0].physical;
                        extent_mirror_num = bbio->mirror_num;
                        extent_dev = bbio->stripes[0].dev;
                        btrfs_put_bbio(bbio);

                        ret = btrfs_lookup_csums_range(csum_root,
                                                extent_logical,
                                                extent_logical + extent_len - 1,
                                                &sctx->csum_list, 1);
                        if (ret)
                                goto out;

                        ret = scrub_extent_for_parity(sparity, extent_logical,
                                                      extent_len,
                                                      extent_physical,
                                                      extent_dev, flags,
                                                      generation,
                                                      extent_mirror_num);

                        scrub_free_csums(sctx);

                        if (ret)
                                goto out;

                        if (extent_logical + extent_len <
                            key.objectid + bytes) {
                                logic_start += map->stripe_len;

                                if (logic_start >= logic_end) {
                                        stop_loop = 1;
                                        break;
                                }

                                if (logic_start < key.objectid + bytes) {
                                        cond_resched();
                                        goto again;
                                }
                        }
next:
                        path->slots[0]++;
                }

                btrfs_release_path(path);

                if (stop_loop)
                        break;

                logic_start += map->stripe_len;
        }
out:
        if (ret < 0)
                scrub_parity_mark_sectors_error(sparity, logic_start,
                                                logic_end - logic_start);
        scrub_parity_put(sparity);
        scrub_submit(sctx);
        mutex_lock(&sctx->wr_lock);
        scrub_wr_submit(sctx);
        mutex_unlock(&sctx->wr_lock);

        btrfs_release_path(path);
        return ret < 0 ? ret : 0;
}

static noinline_for_stack int scrub_stripe(struct scrub_ctx *sctx,
                                           struct map_lookup *map,
                                           struct btrfs_device *scrub_dev,
                                           int num, u64 base, u64 length,
                                           int is_dev_replace)
{
        struct btrfs_path *path, *ppath;
        struct btrfs_fs_info *fs_info = sctx->fs_info;
        struct btrfs_root *root = fs_info->extent_root;
        struct btrfs_root *csum_root = fs_info->csum_root;
        struct btrfs_extent_item *extent;
        struct blk_plug plug;
        u64 flags;
        int ret;
        int slot;
        u64 nstripes;
        struct extent_buffer *l;
        u64 physical;
        u64 logical;
        u64 logic_end;
        u64 physical_end;
        u64 generation;
        int mirror_num;
        struct reada_control *reada1;
        struct reada_control *reada2;
        struct btrfs_key key;
        struct btrfs_key key_end;
        u64 increment = map->stripe_len;
        u64 offset;
        u64 extent_logical;
        u64 extent_physical;
        u64 extent_len;
        u64 stripe_logical;
        u64 stripe_end;
        struct btrfs_device *extent_dev;
        int extent_mirror_num;
        int stop_loop = 0;

        physical = map->stripes[num].physical;
        offset = 0;
        nstripes = div64_u64(length, map->stripe_len);
        if (map->type & BTRFS_BLOCK_GROUP_RAID0) {
                offset = map->stripe_len * num;
                increment = map->stripe_len * map->num_stripes;
                mirror_num = 1;
        } else if (map->type & BTRFS_BLOCK_GROUP_RAID10) {
                int factor = map->num_stripes / map->sub_stripes;
                offset = map->stripe_len * (num / map->sub_stripes);
                increment = map->stripe_len * factor;
                mirror_num = num % map->sub_stripes + 1;
        } else if (map->type & BTRFS_BLOCK_GROUP_RAID1) {
                increment = map->stripe_len;
                mirror_num = num % map->num_stripes + 1;
        } else if (map->type & BTRFS_BLOCK_GROUP_DUP) {
                increment = map->stripe_len;
                mirror_num = num % map->num_stripes + 1;
        } else if (map->type & BTRFS_BLOCK_GROUP_RAID56_MASK) {
                get_raid56_logic_offset(physical, num, map, &offset, NULL);
                increment = map->stripe_len * nr_data_stripes(map);
                mirror_num = 1;
        } else {
                increment = map->stripe_len;
                mirror_num = 1;
        }

        path = btrfs_alloc_path();
        if (!path)
                return -ENOMEM;

        ppath = btrfs_alloc_path();
        if (!ppath) {
                btrfs_free_path(path);
                return -ENOMEM;
        }

        /*
         * work on commit root. The related disk blocks are static as
         * long as COW is applied. This means, it is save to rewrite
         * them to repair disk errors without any race conditions
         */
        path->search_commit_root = 1;
        path->skip_locking = 1;

        ppath->search_commit_root = 1;
        ppath->skip_locking = 1;
        /*
         * trigger the readahead for extent tree csum tree and wait for
         * completion. During readahead, the scrub is officially paused
         * to not hold off transaction commits
         */
        logical = base + offset;
        physical_end = physical + nstripes * map->stripe_len;
        if (map->type & BTRFS_BLOCK_GROUP_RAID56_MASK) {
                get_raid56_logic_offset(physical_end, num,
                                        map, &logic_end, NULL);
                logic_end += base;
        } else {
                logic_end = logical + increment * nstripes;
        }
        wait_event(sctx->list_wait,
                   atomic_read(&sctx->bios_in_flight) == 0);
        scrub_blocked_if_needed(fs_info);

        /* FIXME it might be better to start readahead at commit root */
        key.objectid = logical;
        key.type = BTRFS_EXTENT_ITEM_KEY;
        key.offset = (u64)0;
        key_end.objectid = logic_end;
        key_end.type = BTRFS_METADATA_ITEM_KEY;
        key_end.offset = (u64)-1;
        reada1 = btrfs_reada_add(root, &key, &key_end);

        key.objectid = BTRFS_EXTENT_CSUM_OBJECTID;
        key.type = BTRFS_EXTENT_CSUM_KEY;
        key.offset = logical;
        key_end.objectid = BTRFS_EXTENT_CSUM_OBJECTID;
        key_end.type = BTRFS_EXTENT_CSUM_KEY;
        key_end.offset = logic_end;
        reada2 = btrfs_reada_add(csum_root, &key, &key_end);

        if (!IS_ERR(reada1))
                btrfs_reada_wait(reada1);
        if (!IS_ERR(reada2))
                btrfs_reada_wait(reada2);


        /*
         * collect all data csums for the stripe to avoid seeking during
         * the scrub. This might currently (crc32) end up to be about 1MB
         */
        blk_start_plug(&plug);

        /*
         * now find all extents for each stripe and scrub them
         */
        ret = 0;
        while (physical < physical_end) {
                /*
                 * canceled?
                 */
                if (atomic_read(&fs_info->scrub_cancel_req) ||
                    atomic_read(&sctx->cancel_req)) {
                        ret = -ECANCELED;
                        goto out;
                }
                /*
                 * check to see if we have to pause
                 */
                if (atomic_read(&fs_info->scrub_pause_req)) {
                        /* push queued extents */
                        sctx->flush_all_writes = true;
                        scrub_submit(sctx);
                        mutex_lock(&sctx->wr_lock);
                        scrub_wr_submit(sctx);
                        mutex_unlock(&sctx->wr_lock);
                        wait_event(sctx->list_wait,
                                   atomic_read(&sctx->bios_in_flight) == 0);
                        sctx->flush_all_writes = false;
                        scrub_blocked_if_needed(fs_info);
                }

                if (map->type & BTRFS_BLOCK_GROUP_RAID56_MASK) {
                        ret = get_raid56_logic_offset(physical, num, map,
                                                      &logical,
                                                      &stripe_logical);
                        logical += base;
                        if (ret) {
                                /* it is parity strip */
                                stripe_logical += base;
                                stripe_end = stripe_logical + increment;
                                ret = scrub_raid56_parity(sctx, map, scrub_dev,
                                                          ppath, stripe_logical,
                                                          stripe_end);
                                if (ret)
                                        goto out;
                                goto skip;
                        }
                }

                if (btrfs_fs_incompat(fs_info, SKINNY_METADATA))
                        key.type = BTRFS_METADATA_ITEM_KEY;
                else
                        key.type = BTRFS_EXTENT_ITEM_KEY;
                key.objectid = logical;
                key.offset = (u64)-1;

                ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
                if (ret < 0)
                        goto out;

                if (ret > 0) {
                        ret = btrfs_previous_extent_item(root, path, 0);
                        if (ret < 0)
                                goto out;
                        if (ret > 0) {
                                /* there's no smaller item, so stick with the
                                 * larger one */
                                btrfs_release_path(path);
                                ret = btrfs_search_slot(NULL, root, &key,
                                                        path, 0, 0);
                                if (ret < 0)
                                        goto out;
                        }
                }

                stop_loop = 0;
                while (1) {
                        u64 bytes;

                        l = path->nodes[0];
                        slot = path->slots[0];
                        if (slot >= btrfs_header_nritems(l)) {
                                ret = btrfs_next_leaf(root, path);
                                if (ret == 0)
                                        continue;
                                if (ret < 0)
                                        goto out;

                                stop_loop = 1;
                                break;
                        }
                        btrfs_item_key_to_cpu(l, &key, slot);

                        if (key.type != BTRFS_EXTENT_ITEM_KEY &&
                            key.type != BTRFS_METADATA_ITEM_KEY)
                                goto next;

                        if (key.type == BTRFS_METADATA_ITEM_KEY)
                                bytes = fs_info->nodesize;
                        else
                                bytes = key.offset;

                        if (key.objectid + bytes <= logical)
                                goto next;

                        if (key.objectid >= logical + map->stripe_len) {
                                /* out of this device extent */
                                if (key.objectid >= logic_end)
                                        stop_loop = 1;
                                break;
                        }

                        extent = btrfs_item_ptr(l, slot,
                                                struct btrfs_extent_item);
                        flags = btrfs_extent_flags(l, extent);
                        generation = btrfs_extent_generation(l, extent);

                        if ((flags & BTRFS_EXTENT_FLAG_TREE_BLOCK) &&
                            (key.objectid < logical ||
                             key.objectid + bytes >
                             logical + map->stripe_len)) {
                                btrfs_err(fs_info,
                                           "scrub: tree block %llu spanning stripes, ignored. logical=%llu",
                                       key.objectid, logical);
                                spin_lock(&sctx->stat_lock);
                                sctx->stat.uncorrectable_errors++;
                                spin_unlock(&sctx->stat_lock);
                                goto next;
                        }

again:
                        extent_logical = key.objectid;
                        extent_len = bytes;

                        /*
                         * trim extent to this stripe
                         */
                        if (extent_logical < logical) {
                                extent_len -= logical - extent_logical;
                                extent_logical = logical;
                        }
                        if (extent_logical + extent_len >
                            logical + map->stripe_len) {
                                extent_len = logical + map->stripe_len -
                                             extent_logical;
                        }

                        extent_physical = extent_logical - logical + physical;
                        extent_dev = scrub_dev;
                        extent_mirror_num = mirror_num;
                        if (is_dev_replace)
                                scrub_remap_extent(fs_info, extent_logical,
                                                   extent_len, &extent_physical,
                                                   &extent_dev,
                                                   &extent_mirror_num);

                        ret = btrfs_lookup_csums_range(csum_root,
                                                       extent_logical,
                                                       extent_logical +
                                                       extent_len - 1,
                                                       &sctx->csum_list, 1);
                        if (ret)
                                goto out;

                        ret = scrub_extent(sctx, extent_logical, extent_len,
                                           extent_physical, extent_dev, flags,
                                           generation, extent_mirror_num,
                                           extent_logical - logical + physical);

                        scrub_free_csums(sctx);

                        if (ret)
                                goto out;

                        if (extent_logical + extent_len <
                            key.objectid + bytes) {
                                if (map->type & BTRFS_BLOCK_GROUP_RAID56_MASK) {
                                        /*
                                         * loop until we find next data stripe
                                         * or we have finished all stripes.
                                         */
loop:
                                        physical += map->stripe_len;
                                        ret = get_raid56_logic_offset(physical,
                                                        num, map, &logical,
                                                        &stripe_logical);
                                        logical += base;

                                        if (ret && physical < physical_end) {
                                                stripe_logical += base;
                                                stripe_end = stripe_logical +
                                                                increment;
                                                ret = scrub_raid56_parity(sctx,
                                                        map, scrub_dev, ppath,
                                                        stripe_logical,
                                                        stripe_end);
                                                if (ret)
                                                        goto out;
                                                goto loop;
                                        }
                                } else {
                                        physical += map->stripe_len;
                                        logical += increment;
                                }
                                if (logical < key.objectid + bytes) {
                                        cond_resched();
                                        goto again;
                                }

                                if (physical >= physical_end) {
                                        stop_loop = 1;
                                        break;
                                }
                        }
next:
                        path->slots[0]++;
                }
                btrfs_release_path(path);
skip:
                logical += increment;
                physical += map->stripe_len;
                spin_lock(&sctx->stat_lock);
                if (stop_loop)
                        sctx->stat.last_physical = map->stripes[num].physical +
                                                   length;
                else
                        sctx->stat.last_physical = physical;
                spin_unlock(&sctx->stat_lock);
                if (stop_loop)
                        break;
        }
out:
        /* push queued extents */
        scrub_submit(sctx);
        mutex_lock(&sctx->wr_lock);
        scrub_wr_submit(sctx);
        mutex_unlock(&sctx->wr_lock);

        blk_finish_plug(&plug);
        btrfs_free_path(path);
        btrfs_free_path(ppath);
        return ret < 0 ? ret : 0;
}

static noinline_for_stack int scrub_chunk(struct scrub_ctx *sctx,
                                          struct btrfs_device *scrub_dev,
                                          u64 chunk_offset, u64 length,
                                          u64 dev_offset,
                                          struct btrfs_block_group_cache *cache,
                                          int is_dev_replace)
{
        struct btrfs_fs_info *fs_info = sctx->fs_info;
        struct btrfs_mapping_tree *map_tree = &fs_info->mapping_tree;
        struct map_lookup *map;
        struct extent_map *em;
        int i;
        int ret = 0;

        read_lock(&map_tree->map_tree.lock);
        em = lookup_extent_mapping(&map_tree->map_tree, chunk_offset, 1);
        read_unlock(&map_tree->map_tree.lock);

        if (!em) {
                /*
                 * Might have been an unused block group deleted by the cleaner
                 * kthread or relocation.
                 */
                spin_lock(&cache->lock);
                if (!cache->removed)
                        ret = -EINVAL;
                spin_unlock(&cache->lock);

                return ret;
        }

        map = em->map_lookup;
        if (em->start != chunk_offset)
                goto out;

        if (em->len < length)
                goto out;

        for (i = 0; i < map->num_stripes; ++i) {
                if (map->stripes[i].dev->bdev == scrub_dev->bdev &&
                    map->stripes[i].physical == dev_offset) {
                        ret = scrub_stripe(sctx, map, scrub_dev, i,
                                           chunk_offset, length,
                                           is_dev_replace);
                        if (ret)
                                goto out;
                }
        }
out:
        free_extent_map(em);

        return ret;
}

static noinline_for_stack
int scrub_enumerate_chunks(struct scrub_ctx *sctx,
                           struct btrfs_device *scrub_dev, u64 start, u64 end,
                           int is_dev_replace)
{
        struct btrfs_dev_extent *dev_extent = NULL;
        struct btrfs_path *path;
        struct btrfs_fs_info *fs_info = sctx->fs_info;
        struct btrfs_root *root = fs_info->dev_root;
        u64 length;
        u64 chunk_offset;
        int ret = 0;
        int ro_set;
        int slot;
        struct extent_buffer *l;
        struct btrfs_key key;
        struct btrfs_key found_key;
        struct btrfs_block_group_cache *cache;
        struct btrfs_dev_replace *dev_replace = &fs_info->dev_replace;

        path = btrfs_alloc_path();
        if (!path)
                return -ENOMEM;

        path->reada = READA_FORWARD;
        path->search_commit_root = 1;
        path->skip_locking = 1;

        key.objectid = scrub_dev->devid;
        key.offset = 0ull;
        key.type = BTRFS_DEV_EXTENT_KEY;

        while (1) {
                ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
                if (ret < 0)
                        break;
                if (ret > 0) {
                        if (path->slots[0] >=
                            btrfs_header_nritems(path->nodes[0])) {
                                ret = btrfs_next_leaf(root, path);
                                if (ret < 0)
                                        break;
                                if (ret > 0) {
                                        ret = 0;
                                        break;
                                }
                        } else {
                                ret = 0;
                        }
                }

                l = path->nodes[0];
                slot = path->slots[0];

                btrfs_item_key_to_cpu(l, &found_key, slot);

                if (found_key.objectid != scrub_dev->devid)
                        break;

                if (found_key.type != BTRFS_DEV_EXTENT_KEY)
                        break;

                if (found_key.offset >= end)
                        break;

                if (found_key.offset < key.offset)
                        break;

                dev_extent = btrfs_item_ptr(l, slot, struct btrfs_dev_extent);
                length = btrfs_dev_extent_length(l, dev_extent);

                if (found_key.offset + length <= start)
                        goto skip;

                chunk_offset = btrfs_dev_extent_chunk_offset(l, dev_extent);

                /*
                 * get a reference on the corresponding block group to prevent
                 * the chunk from going away while we scrub it
                 */
                cache = btrfs_lookup_block_group(fs_info, chunk_offset);

                /* some chunks are removed but not committed to disk yet,
                 * continue scrubbing */
                if (!cache)
                        goto skip;

                /*
                 * we need call btrfs_inc_block_group_ro() with scrubs_paused,
                 * to avoid deadlock caused by:
                 * btrfs_inc_block_group_ro()
                 * -> btrfs_wait_for_commit()
                 * -> btrfs_commit_transaction()
                 * -> btrfs_scrub_pause()
                 */
                scrub_pause_on(fs_info);
                ret = btrfs_inc_block_group_ro(fs_info, cache);
                if (!ret && is_dev_replace) {
                        /*
                         * If we are doing a device replace wait for any tasks
                         * that started dellaloc right before we set the block
                         * group to RO mode, as they might have just allocated
                         * an extent from it or decided they could do a nocow
                         * write. And if any such tasks did that, wait for their
                         * ordered extents to complete and then commit the
                         * current transaction, so that we can later see the new
                         * extent items in the extent tree - the ordered extents
                         * create delayed data references (for cow writes) when
                         * they complete, which will be run and insert the
                         * corresponding extent items into the extent tree when
                         * we commit the transaction they used when running
                         * inode.c:btrfs_finish_ordered_io(). We later use
                         * the commit root of the extent tree to find extents
                         * to copy from the srcdev into the tgtdev, and we don't
                         * want to miss any new extents.
                         */
                        btrfs_wait_block_group_reservations(cache);
                        btrfs_wait_nocow_writers(cache);
                        ret = btrfs_wait_ordered_roots(fs_info, U64_MAX,
                                                       cache->key.objectid,
                                                       cache->key.offset);
                        if (ret > 0) {
                                struct btrfs_trans_handle *trans;

                                trans = btrfs_join_transaction(root);
                                if (IS_ERR(trans))
                                        ret = PTR_ERR(trans);
                                else
                                        ret = btrfs_commit_transaction(trans);
                                if (ret) {
                                        scrub_pause_off(fs_info);
                                        btrfs_put_block_group(cache);
                                        break;
                                }
                        }
                }
                scrub_pause_off(fs_info);

                if (ret == 0) {
                        ro_set = 1;
                } else if (ret == -ENOSPC) {
                        /*
                         * btrfs_inc_block_group_ro return -ENOSPC when it
                         * failed in creating new chunk for metadata.
                         * It is not a problem for scrub/replace, because
                         * metadata are always cowed, and our scrub paused
                         * commit_transactions.
                         */
                        ro_set = 0;
                } else {
                        btrfs_warn(fs_info,
                                   "failed setting block group ro: %d", ret);
                        btrfs_put_block_group(cache);
                        break;
                }

                btrfs_dev_replace_lock(&fs_info->dev_replace, 1);
                dev_replace->cursor_right = found_key.offset + length;
                dev_replace->cursor_left = found_key.offset;
                dev_replace->item_needs_writeback = 1;
                btrfs_dev_replace_unlock(&fs_info->dev_replace, 1);
                ret = scrub_chunk(sctx, scrub_dev, chunk_offset, length,
                                  found_key.offset, cache, is_dev_replace);

                /*
                 * flush, submit all pending read and write bios, afterwards
                 * wait for them.
                 * Note that in the dev replace case, a read request causes
                 * write requests that are submitted in the read completion
                 * worker. Therefore in the current situation, it is required
                 * that all write requests are flushed, so that all read and
                 * write requests are really completed when bios_in_flight
                 * changes to 0.
                 */
                sctx->flush_all_writes = true;
                scrub_submit(sctx);
                mutex_lock(&sctx->wr_lock);
                scrub_wr_submit(sctx);
                mutex_unlock(&sctx->wr_lock);

                wait_event(sctx->list_wait,
                           atomic_read(&sctx->bios_in_flight) == 0);

                scrub_pause_on(fs_info);

                /*
                 * must be called before we decrease @scrub_paused.
                 * make sure we don't block transaction commit while
                 * we are waiting pending workers finished.
                 */
                wait_event(sctx->list_wait,
                           atomic_read(&sctx->workers_pending) == 0);
                sctx->flush_all_writes = false;

                scrub_pause_off(fs_info);

                btrfs_dev_replace_lock(&fs_info->dev_replace, 1);
                dev_replace->cursor_left = dev_replace->cursor_right;
                dev_replace->item_needs_writeback = 1;
                btrfs_dev_replace_unlock(&fs_info->dev_replace, 1);

                if (ro_set)
                        btrfs_dec_block_group_ro(cache);

                /*
                 * We might have prevented the cleaner kthread from deleting
                 * this block group if it was already unused because we raced
                 * and set it to RO mode first. So add it back to the unused
                 * list, otherwise it might not ever be deleted unless a manual
                 * balance is triggered or it becomes used and unused again.
                 */
                spin_lock(&cache->lock);
                if (!cache->removed && !cache->ro && cache->reserved == 0 &&
                    btrfs_block_group_used(&cache->item) == 0) {
                        spin_unlock(&cache->lock);
                        spin_lock(&fs_info->unused_bgs_lock);
                        if (list_empty(&cache->bg_list)) {
                                btrfs_get_block_group(cache);
                                list_add_tail(&cache->bg_list,
                                              &fs_info->unused_bgs);
                        }
                        spin_unlock(&fs_info->unused_bgs_lock);
                } else {
                        spin_unlock(&cache->lock);
                }

                btrfs_put_block_group(cache);
                if (ret)
                        break;
                if (is_dev_replace &&
                    atomic64_read(&dev_replace->num_write_errors) > 0) {
                        ret = -EIO;
                        break;
                }
                if (sctx->stat.malloc_errors > 0) {
                        ret = -ENOMEM;
                        break;
                }
skip:
                key.offset = found_key.offset + length;
                btrfs_release_path(path);
        }

        btrfs_free_path(path);

        return ret;
}

static noinline_for_stack int scrub_supers(struct scrub_ctx *sctx,
                                           struct btrfs_device *scrub_dev)
{
        int     i;
        u64     bytenr;
        u64     gen;
        int     ret;
        struct btrfs_fs_info *fs_info = sctx->fs_info;

        if (test_bit(BTRFS_FS_STATE_ERROR, &fs_info->fs_state))
                return -EIO;

        /* Seed devices of a new filesystem has their own generation. */
        if (scrub_dev->fs_devices != fs_info->fs_devices)
                gen = scrub_dev->generation;
        else
                gen = fs_info->last_trans_committed;

        for (i = 0; i < BTRFS_SUPER_MIRROR_MAX; i++) {
                bytenr = btrfs_sb_offset(i);
                if (bytenr + BTRFS_SUPER_INFO_SIZE >
                    scrub_dev->commit_total_bytes)
                        break;

                ret = scrub_pages(sctx, bytenr, BTRFS_SUPER_INFO_SIZE, bytenr,
                                  scrub_dev, BTRFS_EXTENT_FLAG_SUPER, gen, i,
                                  NULL, 1, bytenr);
                if (ret)
                        return ret;
        }
        wait_event(sctx->list_wait, atomic_read(&sctx->bios_in_flight) == 0);

        return 0;
}

/*
 * get a reference count on fs_info->scrub_workers. start worker if necessary
 */
static noinline_for_stack int scrub_workers_get(struct btrfs_fs_info *fs_info,
                                                int is_dev_replace)
{
        unsigned int flags = WQ_FREEZABLE | WQ_UNBOUND;
        int max_active = fs_info->thread_pool_size;

        if (fs_info->scrub_workers_refcnt == 0) {
                fs_info->scrub_workers = btrfs_alloc_workqueue(fs_info, "scrub",
                                flags, is_dev_replace ? 1 : max_active, 4);
                if (!fs_info->scrub_workers)
                        goto fail_scrub_workers;

                fs_info->scrub_wr_completion_workers =
                        btrfs_alloc_workqueue(fs_info, "scrubwrc", flags,
                                              max_active, 2);
                if (!fs_info->scrub_wr_completion_workers)
                        goto fail_scrub_wr_completion_workers;

                fs_info->scrub_nocow_workers =
                        btrfs_alloc_workqueue(fs_info, "scrubnc", flags, 1, 0);
                if (!fs_info->scrub_nocow_workers)
                        goto fail_scrub_nocow_workers;
                fs_info->scrub_parity_workers =
                        btrfs_alloc_workqueue(fs_info, "scrubparity", flags,
                                              max_active, 2);
                if (!fs_info->scrub_parity_workers)
                        goto fail_scrub_parity_workers;
        }
        ++fs_info->scrub_workers_refcnt;
        return 0;

fail_scrub_parity_workers:
        btrfs_destroy_workqueue(fs_info->scrub_nocow_workers);
fail_scrub_nocow_workers:
        btrfs_destroy_workqueue(fs_info->scrub_wr_completion_workers);
fail_scrub_wr_completion_workers:
        btrfs_destroy_workqueue(fs_info->scrub_workers);
fail_scrub_workers:
        return -ENOMEM;
}

static noinline_for_stack void scrub_workers_put(struct btrfs_fs_info *fs_info)
{
        if (--fs_info->scrub_workers_refcnt == 0) {
                btrfs_destroy_workqueue(fs_info->scrub_workers);
                btrfs_destroy_workqueue(fs_info->scrub_wr_completion_workers);
                btrfs_destroy_workqueue(fs_info->scrub_nocow_workers);
                btrfs_destroy_workqueue(fs_info->scrub_parity_workers);
        }
        WARN_ON(fs_info->scrub_workers_refcnt < 0);
}

int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
                    u64 end, struct btrfs_scrub_progress *progress,
                    int readonly, int is_dev_replace)
{
        struct scrub_ctx *sctx;
        int ret;
        struct btrfs_device *dev;
        struct rcu_string *name;

        if (btrfs_fs_closing(fs_info))
                return -EINVAL;

        if (fs_info->nodesize > BTRFS_STRIPE_LEN) {
                /*
                 * in this case scrub is unable to calculate the checksum
                 * the way scrub is implemented. Do not handle this
                 * situation at all because it won't ever happen.
                 */
                btrfs_err(fs_info,
                           "scrub: size assumption nodesize <= BTRFS_STRIPE_LEN (%d <= %d) fails",
                       fs_info->nodesize,
                       BTRFS_STRIPE_LEN);
                return -EINVAL;
        }

        if (fs_info->sectorsize != PAGE_SIZE) {
                /* not supported for data w/o checksums */
                btrfs_err_rl(fs_info,
                           "scrub: size assumption sectorsize != PAGE_SIZE (%d != %lu) fails",
                       fs_info->sectorsize, PAGE_SIZE);
                return -EINVAL;
        }

        if (fs_info->nodesize >
            PAGE_SIZE * SCRUB_MAX_PAGES_PER_BLOCK ||
            fs_info->sectorsize > PAGE_SIZE * SCRUB_MAX_PAGES_PER_BLOCK) {
                /*
                 * would exhaust the array bounds of pagev member in
                 * struct scrub_block
                 */
                btrfs_err(fs_info,
                          "scrub: size assumption nodesize and sectorsize <= SCRUB_MAX_PAGES_PER_BLOCK (%d <= %d && %d <= %d) fails",
                       fs_info->nodesize,
                       SCRUB_MAX_PAGES_PER_BLOCK,
                       fs_info->sectorsize,
                       SCRUB_MAX_PAGES_PER_BLOCK);
                return -EINVAL;
        }


        mutex_lock(&fs_info->fs_devices->device_list_mutex);
        dev = btrfs_find_device(fs_info, devid, NULL, NULL);
        if (!dev || (dev->missing && !is_dev_replace)) {
                mutex_unlock(&fs_info->fs_devices->device_list_mutex);
                return -ENODEV;
        }

        if (!is_dev_replace && !readonly && !dev->writeable) {
                mutex_unlock(&fs_info->fs_devices->device_list_mutex);
                rcu_read_lock();
                name = rcu_dereference(dev->name);
                btrfs_err(fs_info, "scrub: device %s is not writable",
                          name->str);
                rcu_read_unlock();
                return -EROFS;
        }

        mutex_lock(&fs_info->scrub_lock);
        if (!dev->in_fs_metadata || dev->is_tgtdev_for_dev_replace) {
                mutex_unlock(&fs_info->scrub_lock);
                mutex_unlock(&fs_info->fs_devices->device_list_mutex);
                return -EIO;
        }

        btrfs_dev_replace_lock(&fs_info->dev_replace, 0);
        if (dev->scrub_device ||
            (!is_dev_replace &&
             btrfs_dev_replace_is_ongoing(&fs_info->dev_replace))) {
                btrfs_dev_replace_unlock(&fs_info->dev_replace, 0);
                mutex_unlock(&fs_info->scrub_lock);
                mutex_unlock(&fs_info->fs_devices->device_list_mutex);
                return -EINPROGRESS;
        }
        btrfs_dev_replace_unlock(&fs_info->dev_replace, 0);

        ret = scrub_workers_get(fs_info, is_dev_replace);
        if (ret) {
                mutex_unlock(&fs_info->scrub_lock);
                mutex_unlock(&fs_info->fs_devices->device_list_mutex);
                return ret;
        }

        sctx = scrub_setup_ctx(dev, is_dev_replace);
        if (IS_ERR(sctx)) {
                mutex_unlock(&fs_info->scrub_lock);
                mutex_unlock(&fs_info->fs_devices->device_list_mutex);
                scrub_workers_put(fs_info);
                return PTR_ERR(sctx);
        }
        sctx->readonly = readonly;
        dev->scrub_device = sctx;
        mutex_unlock(&fs_info->fs_devices->device_list_mutex);

        /*
         * checking @scrub_pause_req here, we can avoid
         * race between committing transaction and scrubbing.
         */
        __scrub_blocked_if_needed(fs_info);
        atomic_inc(&fs_info->scrubs_running);
        mutex_unlock(&fs_info->scrub_lock);

        if (!is_dev_replace) {
                /*
                 * by holding device list mutex, we can
                 * kick off writing super in log tree sync.
                 */
                mutex_lock(&fs_info->fs_devices->device_list_mutex);
                ret = scrub_supers(sctx, dev);
                mutex_unlock(&fs_info->fs_devices->device_list_mutex);
        }

        if (!ret)
                ret = scrub_enumerate_chunks(sctx, dev, start, end,
                                             is_dev_replace);

        wait_event(sctx->list_wait, atomic_read(&sctx->bios_in_flight) == 0);
        atomic_dec(&fs_info->scrubs_running);
        wake_up(&fs_info->scrub_pause_wait);

        wait_event(sctx->list_wait, atomic_read(&sctx->workers_pending) == 0);

        if (progress)
                memcpy(progress, &sctx->stat, sizeof(*progress));

        mutex_lock(&fs_info->scrub_lock);
        dev->scrub_device = NULL;
        scrub_workers_put(fs_info);
        mutex_unlock(&fs_info->scrub_lock);

        scrub_put_ctx(sctx);

        return ret;
}

void btrfs_scrub_pause(struct btrfs_fs_info *fs_info)
{
        mutex_lock(&fs_info->scrub_lock);
        atomic_inc(&fs_info->scrub_pause_req);
        while (atomic_read(&fs_info->scrubs_paused) !=
               atomic_read(&fs_info->scrubs_running)) {
                mutex_unlock(&fs_info->scrub_lock);
                wait_event(fs_info->scrub_pause_wait,
                           atomic_read(&fs_info->scrubs_paused) ==
                           atomic_read(&fs_info->scrubs_running));
                mutex_lock(&fs_info->scrub_lock);
        }
        mutex_unlock(&fs_info->scrub_lock);
}

void btrfs_scrub_continue(struct btrfs_fs_info *fs_info)
{
        atomic_dec(&fs_info->scrub_pause_req);
        wake_up(&fs_info->scrub_pause_wait);
}

int btrfs_scrub_cancel(struct btrfs_fs_info *fs_info)
{
        mutex_lock(&fs_info->scrub_lock);
        if (!atomic_read(&fs_info->scrubs_running)) {
                mutex_unlock(&fs_info->scrub_lock);
                return -ENOTCONN;
        }

        atomic_inc(&fs_info->scrub_cancel_req);
        while (atomic_read(&fs_info->scrubs_running)) {
                mutex_unlock(&fs_info->scrub_lock);
                wait_event(fs_info->scrub_pause_wait,
                           atomic_read(&fs_info->scrubs_running) == 0);
                mutex_lock(&fs_info->scrub_lock);
        }
        atomic_dec(&fs_info->scrub_cancel_req);
        mutex_unlock(&fs_info->scrub_lock);

        return 0;
}

int btrfs_scrub_cancel_dev(struct btrfs_fs_info *fs_info,
                           struct btrfs_device *dev)
{
        struct scrub_ctx *sctx;

        mutex_lock(&fs_info->scrub_lock);
        sctx = dev->scrub_device;
        if (!sctx) {
                mutex_unlock(&fs_info->scrub_lock);
                return -ENOTCONN;
        }
        atomic_inc(&sctx->cancel_req);
        while (dev->scrub_device) {
                mutex_unlock(&fs_info->scrub_lock);
                wait_event(fs_info->scrub_pause_wait,
                           dev->scrub_device == NULL);
                mutex_lock(&fs_info->scrub_lock);
        }
        mutex_unlock(&fs_info->scrub_lock);

        return 0;
}

int btrfs_scrub_progress(struct btrfs_fs_info *fs_info, u64 devid,
                         struct btrfs_scrub_progress *progress)
{
        struct btrfs_device *dev;
        struct scrub_ctx *sctx = NULL;

        mutex_lock(&fs_info->fs_devices->device_list_mutex);
        dev = btrfs_find_device(fs_info, devid, NULL, NULL);
        if (dev)
                sctx = dev->scrub_device;
        if (sctx)
                memcpy(progress, &sctx->stat, sizeof(*progress));
        mutex_unlock(&fs_info->fs_devices->device_list_mutex);

        return dev ? (sctx ? 0 : -ENOTCONN) : -ENODEV;
}

static void scrub_remap_extent(struct btrfs_fs_info *fs_info,
                               u64 extent_logical, u64 extent_len,
                               u64 *extent_physical,
                               struct btrfs_device **extent_dev,
                               int *extent_mirror_num)
{
        u64 mapped_length;
        struct btrfs_bio *bbio = NULL;
        int ret;

        mapped_length = extent_len;
        ret = btrfs_map_block(fs_info, BTRFS_MAP_READ, extent_logical,
                              &mapped_length, &bbio, 0);
        if (ret || !bbio || mapped_length < extent_len ||
            !bbio->stripes[0].dev->bdev) {
                btrfs_put_bbio(bbio);
                return;
        }

        *extent_physical = bbio->stripes[0].physical;
        *extent_mirror_num = bbio->mirror_num;
        *extent_dev = bbio->stripes[0].dev;
        btrfs_put_bbio(bbio);
}

static int copy_nocow_pages(struct scrub_ctx *sctx, u64 logical, u64 len,
                            int mirror_num, u64 physical_for_dev_replace)
{
        struct scrub_copy_nocow_ctx *nocow_ctx;
        struct btrfs_fs_info *fs_info = sctx->fs_info;

        nocow_ctx = kzalloc(sizeof(*nocow_ctx), GFP_NOFS);
        if (!nocow_ctx) {
                spin_lock(&sctx->stat_lock);
                sctx->stat.malloc_errors++;
                spin_unlock(&sctx->stat_lock);
                return -ENOMEM;
        }

        scrub_pending_trans_workers_inc(sctx);

        nocow_ctx->sctx = sctx;
        nocow_ctx->logical = logical;
        nocow_ctx->len = len;
        nocow_ctx->mirror_num = mirror_num;
        nocow_ctx->physical_for_dev_replace = physical_for_dev_replace;
        btrfs_init_work(&nocow_ctx->work, btrfs_scrubnc_helper,
                        copy_nocow_pages_worker, NULL, NULL);
        INIT_LIST_HEAD(&nocow_ctx->inodes);
        btrfs_queue_work(fs_info->scrub_nocow_workers,
                         &nocow_ctx->work);

        return 0;
}

static int record_inode_for_nocow(u64 inum, u64 offset, u64 root, void *ctx)
{
        struct scrub_copy_nocow_ctx *nocow_ctx = ctx;
        struct scrub_nocow_inode *nocow_inode;

        nocow_inode = kzalloc(sizeof(*nocow_inode), GFP_NOFS);
        if (!nocow_inode)
                return -ENOMEM;
        nocow_inode->inum = inum;
        nocow_inode->offset = offset;
        nocow_inode->root = root;
        list_add_tail(&nocow_inode->list, &nocow_ctx->inodes);
        return 0;
}

#define COPY_COMPLETE 1

static void copy_nocow_pages_worker(struct btrfs_work *work)
{
        struct scrub_copy_nocow_ctx *nocow_ctx =
                container_of(work, struct scrub_copy_nocow_ctx, work);
        struct scrub_ctx *sctx = nocow_ctx->sctx;
        struct btrfs_fs_info *fs_info = sctx->fs_info;
        struct btrfs_root *root = fs_info->extent_root;
        u64 logical = nocow_ctx->logical;
        u64 len = nocow_ctx->len;
        int mirror_num = nocow_ctx->mirror_num;
        u64 physical_for_dev_replace = nocow_ctx->physical_for_dev_replace;
        int ret;
        struct btrfs_trans_handle *trans = NULL;
        struct btrfs_path *path;
        int not_written = 0;

        path = btrfs_alloc_path();
        if (!path) {
                spin_lock(&sctx->stat_lock);
                sctx->stat.malloc_errors++;
                spin_unlock(&sctx->stat_lock);
                not_written = 1;
                goto out;
        }

        trans = btrfs_join_transaction(root);
        if (IS_ERR(trans)) {
                not_written = 1;
                goto out;
        }

        ret = iterate_inodes_from_logical(logical, fs_info, path,
                        record_inode_for_nocow, nocow_ctx, false);
        if (ret != 0 && ret != -ENOENT) {
                btrfs_warn(fs_info,
                           "iterate_inodes_from_logical() failed: log %llu, phys %llu, len %llu, mir %u, ret %d",
                           logical, physical_for_dev_replace, len, mirror_num,
                           ret);
                not_written = 1;
                goto out;
        }

        btrfs_end_transaction(trans);
        trans = NULL;
        while (!list_empty(&nocow_ctx->inodes)) {
                struct scrub_nocow_inode *entry;
                entry = list_first_entry(&nocow_ctx->inodes,
                                         struct scrub_nocow_inode,
                                         list);
                list_del_init(&entry->list);
                ret = copy_nocow_pages_for_inode(entry->inum, entry->offset,
                                                 entry->root, nocow_ctx);
                kfree(entry);
                if (ret == COPY_COMPLETE) {
                        ret = 0;
                        break;
                } else if (ret) {
                        break;
                }
        }
out:
        while (!list_empty(&nocow_ctx->inodes)) {
                struct scrub_nocow_inode *entry;
                entry = list_first_entry(&nocow_ctx->inodes,
                                         struct scrub_nocow_inode,
                                         list);
                list_del_init(&entry->list);
                kfree(entry);
        }
        if (trans && !IS_ERR(trans))
                btrfs_end_transaction(trans);
        if (not_written)
                btrfs_dev_replace_stats_inc(&fs_info->dev_replace.
                                            num_uncorrectable_read_errors);

        btrfs_free_path(path);
        kfree(nocow_ctx);

        scrub_pending_trans_workers_dec(sctx);
}

static int check_extent_to_block(struct btrfs_inode *inode, u64 start, u64 len,
                                 u64 logical)
{
        struct extent_state *cached_state = NULL;
        struct btrfs_ordered_extent *ordered;
        struct extent_io_tree *io_tree;
        struct extent_map *em;
        u64 lockstart = start, lockend = start + len - 1;
        int ret = 0;

        io_tree = &inode->io_tree;

        lock_extent_bits(io_tree, lockstart, lockend, &cached_state);
        ordered = btrfs_lookup_ordered_range(inode, lockstart, len);
        if (ordered) {
                btrfs_put_ordered_extent(ordered);
                ret = 1;
                goto out_unlock;
        }

        em = btrfs_get_extent(inode, NULL, 0, start, len, 0);
        if (IS_ERR(em)) {
                ret = PTR_ERR(em);
                goto out_unlock;
        }

        /*
         * This extent does not actually cover the logical extent anymore,
         * move on to the next inode.
         */
        if (em->block_start > logical ||
            em->block_start + em->block_len < logical + len) {
                free_extent_map(em);
                ret = 1;
                goto out_unlock;
        }
        free_extent_map(em);

out_unlock:
        unlock_extent_cached(io_tree, lockstart, lockend, &cached_state,
                             GFP_NOFS);
        return ret;
}

static int copy_nocow_pages_for_inode(u64 inum, u64 offset, u64 root,
                                      struct scrub_copy_nocow_ctx *nocow_ctx)
{
        struct btrfs_fs_info *fs_info = nocow_ctx->sctx->fs_info;
        struct btrfs_key key;
        struct inode *inode;
        struct page *page;
        struct btrfs_root *local_root;
        struct extent_io_tree *io_tree;
        u64 physical_for_dev_replace;
        u64 nocow_ctx_logical;
        u64 len = nocow_ctx->len;
        unsigned long index;
        int srcu_index;
        int ret = 0;
        int err = 0;

        key.objectid = root;
        key.type = BTRFS_ROOT_ITEM_KEY;
        key.offset = (u64)-1;

        srcu_index = srcu_read_lock(&fs_info->subvol_srcu);

        local_root = btrfs_read_fs_root_no_name(fs_info, &key);
        if (IS_ERR(local_root)) {
                srcu_read_unlock(&fs_info->subvol_srcu, srcu_index);
                return PTR_ERR(local_root);
        }

        key.type = BTRFS_INODE_ITEM_KEY;
        key.objectid = inum;
        key.offset = 0;
        inode = btrfs_iget(fs_info->sb, &key, local_root, NULL);
        srcu_read_unlock(&fs_info->subvol_srcu, srcu_index);
        if (IS_ERR(inode))
                return PTR_ERR(inode);

        /* Avoid truncate/dio/punch hole.. */
        inode_lock(inode);
        inode_dio_wait(inode);

        physical_for_dev_replace = nocow_ctx->physical_for_dev_replace;
        io_tree = &BTRFS_I(inode)->io_tree;
        nocow_ctx_logical = nocow_ctx->logical;

        ret = check_extent_to_block(BTRFS_I(inode), offset, len,
                        nocow_ctx_logical);
        if (ret) {
                ret = ret > 0 ? 0 : ret;
                goto out;
        }

        while (len >= PAGE_SIZE) {
                index = offset >> PAGE_SHIFT;
again:
                page = find_or_create_page(inode->i_mapping, index, GFP_NOFS);
                if (!page) {
                        btrfs_err(fs_info, "find_or_create_page() failed");
                        ret = -ENOMEM;
                        goto out;
                }

                if (PageUptodate(page)) {
                        if (PageDirty(page))
                                goto next_page;
                } else {
                        ClearPageError(page);
                        err = extent_read_full_page(io_tree, page,
                                                           btrfs_get_extent,
                                                           nocow_ctx->mirror_num);
                        if (err) {
                                ret = err;
                                goto next_page;
                        }

                        lock_page(page);
                        /*
                         * If the page has been remove from the page cache,
                         * the data on it is meaningless, because it may be
                         * old one, the new data may be written into the new
                         * page in the page cache.
                         */
                        if (page->mapping != inode->i_mapping) {
                                unlock_page(page);
                                put_page(page);
                                goto again;
                        }
                        if (!PageUptodate(page)) {
                                ret = -EIO;
                                goto next_page;
                        }
                }

                ret = check_extent_to_block(BTRFS_I(inode), offset, len,
                                            nocow_ctx_logical);
                if (ret) {
                        ret = ret > 0 ? 0 : ret;
                        goto next_page;
                }

                err = write_page_nocow(nocow_ctx->sctx,
                                       physical_for_dev_replace, page);
                if (err)
                        ret = err;
next_page:
                unlock_page(page);
                put_page(page);

                if (ret)
                        break;

                offset += PAGE_SIZE;
                physical_for_dev_replace += PAGE_SIZE;
                nocow_ctx_logical += PAGE_SIZE;
                len -= PAGE_SIZE;
        }
        ret = COPY_COMPLETE;
out:
        inode_unlock(inode);
        iput(inode);
        return ret;
}

static int write_page_nocow(struct scrub_ctx *sctx,
                            u64 physical_for_dev_replace, struct page *page)
{
        struct bio *bio;
        struct btrfs_device *dev;
        int ret;

        dev = sctx->wr_tgtdev;
        if (!dev)
                return -EIO;
        if (!dev->bdev) {
                btrfs_warn_rl(dev->fs_info,
                        "scrub write_page_nocow(bdev == NULL) is unexpected");
                return -EIO;
        }
        bio = btrfs_io_bio_alloc(1);
        bio->bi_iter.bi_size = 0;
        bio->bi_iter.bi_sector = physical_for_dev_replace >> 9;
        bio_set_dev(bio, dev->bdev);
        bio->bi_opf = REQ_OP_WRITE | REQ_SYNC;
        ret = bio_add_page(bio, page, PAGE_SIZE, 0);
        if (ret != PAGE_SIZE) {
leave_with_eio:
                bio_put(bio);
                btrfs_dev_stat_inc_and_print(dev, BTRFS_DEV_STAT_WRITE_ERRS);
                return -EIO;
        }

        if (btrfsic_submit_bio_wait(bio))
                goto leave_with_eio;

        bio_put(bio);
        return 0;
}