mm/zsmalloc.c: migration can leave pages in ZS_EMPTY indefinitely

[sfrench/cifs-2.6.git] / mm / zsmalloc.c

/*
 * zsmalloc memory allocator
 *
 * Copyright (C) 2011  Nitin Gupta
 * Copyright (C) 2012, 2013 Minchan Kim
 *
 * This code is released using a dual license strategy: BSD/GPL
 * You can choose the license that better fits your requirements.
 *
 * Released under the terms of 3-clause BSD License
 * Released under the terms of GNU General Public License Version 2.0
 */

/*
 * Following is how we use various fields and flags of underlying
 * struct page(s) to form a zspage.
 *
 * Usage of struct page fields:
 *      page->private: points to zspage
 *      page->freelist(index): links together all component pages of a zspage
 *              For the huge page, this is always 0, so we use this field
 *              to store handle.
 *      page->units: first object offset in a subpage of zspage
 *
 * Usage of struct page flags:
 *      PG_private: identifies the first component page
 *      PG_owner_priv_1: identifies the huge component page
 *
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/magic.h>
#include <linux/bitops.h>
#include <linux/errno.h>
#include <linux/highmem.h>
#include <linux/string.h>
#include <linux/slab.h>
#include <asm/tlbflush.h>
#include <asm/pgtable.h>
#include <linux/cpumask.h>
#include <linux/cpu.h>
#include <linux/vmalloc.h>
#include <linux/preempt.h>
#include <linux/spinlock.h>
#include <linux/shrinker.h>
#include <linux/types.h>
#include <linux/debugfs.h>
#include <linux/zsmalloc.h>
#include <linux/zpool.h>
#include <linux/mount.h>
#include <linux/pseudo_fs.h>
#include <linux/migrate.h>
#include <linux/pagemap.h>
#include <linux/fs.h>

#define ZSPAGE_MAGIC    0x58

/*
 * This must be power of 2 and greater than of equal to sizeof(link_free).
 * These two conditions ensure that any 'struct link_free' itself doesn't
 * span more than 1 page which avoids complex case of mapping 2 pages simply
 * to restore link_free pointer values.
 */
#define ZS_ALIGN                8

/*
 * A single 'zspage' is composed of up to 2^N discontiguous 0-order (single)
 * pages. ZS_MAX_ZSPAGE_ORDER defines upper limit on N.
 */
#define ZS_MAX_ZSPAGE_ORDER 2
#define ZS_MAX_PAGES_PER_ZSPAGE (_AC(1, UL) << ZS_MAX_ZSPAGE_ORDER)

#define ZS_HANDLE_SIZE (sizeof(unsigned long))

/*
 * Object location (<PFN>, <obj_idx>) is encoded as
 * as single (unsigned long) handle value.
 *
 * Note that object index <obj_idx> starts from 0.
 *
 * This is made more complicated by various memory models and PAE.
 */

#ifndef MAX_POSSIBLE_PHYSMEM_BITS
#ifdef MAX_PHYSMEM_BITS
#define MAX_POSSIBLE_PHYSMEM_BITS MAX_PHYSMEM_BITS
#else
/*
 * If this definition of MAX_PHYSMEM_BITS is used, OBJ_INDEX_BITS will just
 * be PAGE_SHIFT
 */
#define MAX_POSSIBLE_PHYSMEM_BITS BITS_PER_LONG
#endif
#endif

#define _PFN_BITS               (MAX_POSSIBLE_PHYSMEM_BITS - PAGE_SHIFT)

/*
 * Memory for allocating for handle keeps object position by
 * encoding <page, obj_idx> and the encoded value has a room
 * in least bit(ie, look at obj_to_location).
 * We use the bit to synchronize between object access by
 * user and migration.
 */
#define HANDLE_PIN_BIT  0

/*
 * Head in allocated object should have OBJ_ALLOCATED_TAG
 * to identify the object was allocated or not.
 * It's okay to add the status bit in the least bit because
 * header keeps handle which is 4byte-aligned address so we
 * have room for two bit at least.
 */
#define OBJ_ALLOCATED_TAG 1
#define OBJ_TAG_BITS 1
#define OBJ_INDEX_BITS  (BITS_PER_LONG - _PFN_BITS - OBJ_TAG_BITS)
#define OBJ_INDEX_MASK  ((_AC(1, UL) << OBJ_INDEX_BITS) - 1)

#define FULLNESS_BITS   2
#define CLASS_BITS      8
#define ISOLATED_BITS   3
#define MAGIC_VAL_BITS  8

#define MAX(a, b) ((a) >= (b) ? (a) : (b))
/* ZS_MIN_ALLOC_SIZE must be multiple of ZS_ALIGN */
#define ZS_MIN_ALLOC_SIZE \
        MAX(32, (ZS_MAX_PAGES_PER_ZSPAGE << PAGE_SHIFT >> OBJ_INDEX_BITS))
/* each chunk includes extra space to keep handle */
#define ZS_MAX_ALLOC_SIZE       PAGE_SIZE

/*
 * On systems with 4K page size, this gives 255 size classes! There is a
 * trader-off here:
 *  - Large number of size classes is potentially wasteful as free page are
 *    spread across these classes
 *  - Small number of size classes causes large internal fragmentation
 *  - Probably its better to use specific size classes (empirically
 *    determined). NOTE: all those class sizes must be set as multiple of
 *    ZS_ALIGN to make sure link_free itself never has to span 2 pages.
 *
 *  ZS_MIN_ALLOC_SIZE and ZS_SIZE_CLASS_DELTA must be multiple of ZS_ALIGN
 *  (reason above)
 */
#define ZS_SIZE_CLASS_DELTA     (PAGE_SIZE >> CLASS_BITS)
#define ZS_SIZE_CLASSES (DIV_ROUND_UP(ZS_MAX_ALLOC_SIZE - ZS_MIN_ALLOC_SIZE, \
                                      ZS_SIZE_CLASS_DELTA) + 1)

enum fullness_group {
        ZS_EMPTY,
        ZS_ALMOST_EMPTY,
        ZS_ALMOST_FULL,
        ZS_FULL,
        NR_ZS_FULLNESS,
};

enum zs_stat_type {
        CLASS_EMPTY,
        CLASS_ALMOST_EMPTY,
        CLASS_ALMOST_FULL,
        CLASS_FULL,
        OBJ_ALLOCATED,
        OBJ_USED,
        NR_ZS_STAT_TYPE,
};

struct zs_size_stat {
        unsigned long objs[NR_ZS_STAT_TYPE];
};

#ifdef CONFIG_ZSMALLOC_STAT
static struct dentry *zs_stat_root;
#endif

#ifdef CONFIG_COMPACTION
static struct vfsmount *zsmalloc_mnt;
#endif

/*
 * We assign a page to ZS_ALMOST_EMPTY fullness group when:
 *      n <= N / f, where
 * n = number of allocated objects
 * N = total number of objects zspage can store
 * f = fullness_threshold_frac
 *
 * Similarly, we assign zspage to:
 *      ZS_ALMOST_FULL  when n > N / f
 *      ZS_EMPTY        when n == 0
 *      ZS_FULL         when n == N
 *
 * (see: fix_fullness_group())
 */
static const int fullness_threshold_frac = 4;
static size_t huge_class_size;

struct size_class {
        spinlock_t lock;
        struct list_head fullness_list[NR_ZS_FULLNESS];
        /*
         * Size of objects stored in this class. Must be multiple
         * of ZS_ALIGN.
         */
        int size;
        int objs_per_zspage;
        /* Number of PAGE_SIZE sized pages to combine to form a 'zspage' */
        int pages_per_zspage;

        unsigned int index;
        struct zs_size_stat stats;
};

/* huge object: pages_per_zspage == 1 && maxobj_per_zspage == 1 */
static void SetPageHugeObject(struct page *page)
{
        SetPageOwnerPriv1(page);
}

static void ClearPageHugeObject(struct page *page)
{
        ClearPageOwnerPriv1(page);
}

static int PageHugeObject(struct page *page)
{
        return PageOwnerPriv1(page);
}

/*
 * Placed within free objects to form a singly linked list.
 * For every zspage, zspage->freeobj gives head of this list.
 *
 * This must be power of 2 and less than or equal to ZS_ALIGN
 */
struct link_free {
        union {
                /*
                 * Free object index;
                 * It's valid for non-allocated object
                 */
                unsigned long next;
                /*
                 * Handle of allocated object.
                 */
                unsigned long handle;
        };
};

struct zs_pool {
        const char *name;

        struct size_class *size_class[ZS_SIZE_CLASSES];
        struct kmem_cache *handle_cachep;
        struct kmem_cache *zspage_cachep;

        atomic_long_t pages_allocated;

        struct zs_pool_stats stats;

        /* Compact classes */
        struct shrinker shrinker;

#ifdef CONFIG_ZSMALLOC_STAT
        struct dentry *stat_dentry;
#endif
#ifdef CONFIG_COMPACTION
        struct inode *inode;
        struct work_struct free_work;
#endif
};

struct zspage {
        struct {
                unsigned int fullness:FULLNESS_BITS;
                unsigned int class:CLASS_BITS + 1;
                unsigned int isolated:ISOLATED_BITS;
                unsigned int magic:MAGIC_VAL_BITS;
        };
        unsigned int inuse;
        unsigned int freeobj;
        struct page *first_page;
        struct list_head list; /* fullness list */
#ifdef CONFIG_COMPACTION
        rwlock_t lock;
#endif
};

struct mapping_area {
#ifdef CONFIG_PGTABLE_MAPPING
        struct vm_struct *vm; /* vm area for mapping object that span pages */
#else
        char *vm_buf; /* copy buffer for objects that span pages */
#endif
        char *vm_addr; /* address of kmap_atomic()'ed pages */
        enum zs_mapmode vm_mm; /* mapping mode */
};

#ifdef CONFIG_COMPACTION
static int zs_register_migration(struct zs_pool *pool);
static void zs_unregister_migration(struct zs_pool *pool);
static void migrate_lock_init(struct zspage *zspage);
static void migrate_read_lock(struct zspage *zspage);
static void migrate_read_unlock(struct zspage *zspage);
static void kick_deferred_free(struct zs_pool *pool);
static void init_deferred_free(struct zs_pool *pool);
static void SetZsPageMovable(struct zs_pool *pool, struct zspage *zspage);
#else
static int zsmalloc_mount(void) { return 0; }
static void zsmalloc_unmount(void) {}
static int zs_register_migration(struct zs_pool *pool) { return 0; }
static void zs_unregister_migration(struct zs_pool *pool) {}
static void migrate_lock_init(struct zspage *zspage) {}
static void migrate_read_lock(struct zspage *zspage) {}
static void migrate_read_unlock(struct zspage *zspage) {}
static void kick_deferred_free(struct zs_pool *pool) {}
static void init_deferred_free(struct zs_pool *pool) {}
static void SetZsPageMovable(struct zs_pool *pool, struct zspage *zspage) {}
#endif

static int create_cache(struct zs_pool *pool)
{
        pool->handle_cachep = kmem_cache_create("zs_handle", ZS_HANDLE_SIZE,
                                        0, 0, NULL);
        if (!pool->handle_cachep)
                return 1;

        pool->zspage_cachep = kmem_cache_create("zspage", sizeof(struct zspage),
                                        0, 0, NULL);
        if (!pool->zspage_cachep) {
                kmem_cache_destroy(pool->handle_cachep);
                pool->handle_cachep = NULL;
                return 1;
        }

        return 0;
}

static void destroy_cache(struct zs_pool *pool)
{
        kmem_cache_destroy(pool->handle_cachep);
        kmem_cache_destroy(pool->zspage_cachep);
}

static unsigned long cache_alloc_handle(struct zs_pool *pool, gfp_t gfp)
{
        return (unsigned long)kmem_cache_alloc(pool->handle_cachep,
                        gfp & ~(__GFP_HIGHMEM|__GFP_MOVABLE));
}

static void cache_free_handle(struct zs_pool *pool, unsigned long handle)
{
        kmem_cache_free(pool->handle_cachep, (void *)handle);
}

static struct zspage *cache_alloc_zspage(struct zs_pool *pool, gfp_t flags)
{
        return kmem_cache_alloc(pool->zspage_cachep,
                        flags & ~(__GFP_HIGHMEM|__GFP_MOVABLE));
}

static void cache_free_zspage(struct zs_pool *pool, struct zspage *zspage)
{
        kmem_cache_free(pool->zspage_cachep, zspage);
}

static void record_obj(unsigned long handle, unsigned long obj)
{
        /*
         * lsb of @obj represents handle lock while other bits
         * represent object value the handle is pointing so
         * updating shouldn't do store tearing.
         */
        WRITE_ONCE(*(unsigned long *)handle, obj);
}

/* zpool driver */

#ifdef CONFIG_ZPOOL

static void *zs_zpool_create(const char *name, gfp_t gfp,
                             const struct zpool_ops *zpool_ops,
                             struct zpool *zpool)
{
        /*
         * Ignore global gfp flags: zs_malloc() may be invoked from
         * different contexts and its caller must provide a valid
         * gfp mask.
         */
        return zs_create_pool(name);
}

static void zs_zpool_destroy(void *pool)
{
        zs_destroy_pool(pool);
}

static int zs_zpool_malloc(void *pool, size_t size, gfp_t gfp,
                        unsigned long *handle)
{
        *handle = zs_malloc(pool, size, gfp);
        return *handle ? 0 : -1;
}
static void zs_zpool_free(void *pool, unsigned long handle)
{
        zs_free(pool, handle);
}

static void *zs_zpool_map(void *pool, unsigned long handle,
                        enum zpool_mapmode mm)
{
        enum zs_mapmode zs_mm;

        switch (mm) {
        case ZPOOL_MM_RO:
                zs_mm = ZS_MM_RO;
                break;
        case ZPOOL_MM_WO:
                zs_mm = ZS_MM_WO;
                break;
        case ZPOOL_MM_RW: /* fall through */
        default:
                zs_mm = ZS_MM_RW;
                break;
        }

        return zs_map_object(pool, handle, zs_mm);
}
static void zs_zpool_unmap(void *pool, unsigned long handle)
{
        zs_unmap_object(pool, handle);
}

static u64 zs_zpool_total_size(void *pool)
{
        return zs_get_total_pages(pool) << PAGE_SHIFT;
}

static struct zpool_driver zs_zpool_driver = {
        .type =         "zsmalloc",
        .owner =        THIS_MODULE,
        .create =       zs_zpool_create,
        .destroy =      zs_zpool_destroy,
        .malloc =       zs_zpool_malloc,
        .free =         zs_zpool_free,
        .map =          zs_zpool_map,
        .unmap =        zs_zpool_unmap,
        .total_size =   zs_zpool_total_size,
};

MODULE_ALIAS("zpool-zsmalloc");
#endif /* CONFIG_ZPOOL */

/* per-cpu VM mapping areas for zspage accesses that cross page boundaries */
static DEFINE_PER_CPU(struct mapping_area, zs_map_area);

static bool is_zspage_isolated(struct zspage *zspage)
{
        return zspage->isolated;
}

static __maybe_unused int is_first_page(struct page *page)
{
        return PagePrivate(page);
}

/* Protected by class->lock */
static inline int get_zspage_inuse(struct zspage *zspage)
{
        return zspage->inuse;
}

static inline void set_zspage_inuse(struct zspage *zspage, int val)
{
        zspage->inuse = val;
}

static inline void mod_zspage_inuse(struct zspage *zspage, int val)
{
        zspage->inuse += val;
}

static inline struct page *get_first_page(struct zspage *zspage)
{
        struct page *first_page = zspage->first_page;

        VM_BUG_ON_PAGE(!is_first_page(first_page), first_page);
        return first_page;
}

static inline int get_first_obj_offset(struct page *page)
{
        return page->units;
}

static inline void set_first_obj_offset(struct page *page, int offset)
{
        page->units = offset;
}

static inline unsigned int get_freeobj(struct zspage *zspage)
{
        return zspage->freeobj;
}

static inline void set_freeobj(struct zspage *zspage, unsigned int obj)
{
        zspage->freeobj = obj;
}

static void get_zspage_mapping(struct zspage *zspage,
                                unsigned int *class_idx,
                                enum fullness_group *fullness)
{
        BUG_ON(zspage->magic != ZSPAGE_MAGIC);

        *fullness = zspage->fullness;
        *class_idx = zspage->class;
}

static void set_zspage_mapping(struct zspage *zspage,
                                unsigned int class_idx,
                                enum fullness_group fullness)
{
        zspage->class = class_idx;
        zspage->fullness = fullness;
}

/*
 * zsmalloc divides the pool into various size classes where each
 * class maintains a list of zspages where each zspage is divided
 * into equal sized chunks. Each allocation falls into one of these
 * classes depending on its size. This function returns index of the
 * size class which has chunk size big enough to hold the give size.
 */
static int get_size_class_index(int size)
{
        int idx = 0;

        if (likely(size > ZS_MIN_ALLOC_SIZE))
                idx = DIV_ROUND_UP(size - ZS_MIN_ALLOC_SIZE,
                                ZS_SIZE_CLASS_DELTA);

        return min_t(int, ZS_SIZE_CLASSES - 1, idx);
}

/* type can be of enum type zs_stat_type or fullness_group */
static inline void zs_stat_inc(struct size_class *class,
                                int type, unsigned long cnt)
{
        class->stats.objs[type] += cnt;
}

/* type can be of enum type zs_stat_type or fullness_group */
static inline void zs_stat_dec(struct size_class *class,
                                int type, unsigned long cnt)
{
        class->stats.objs[type] -= cnt;
}

/* type can be of enum type zs_stat_type or fullness_group */
static inline unsigned long zs_stat_get(struct size_class *class,
                                int type)
{
        return class->stats.objs[type];
}

#ifdef CONFIG_ZSMALLOC_STAT

static void __init zs_stat_init(void)
{
        if (!debugfs_initialized()) {
                pr_warn("debugfs not available, stat dir not created\n");
                return;
        }

        zs_stat_root = debugfs_create_dir("zsmalloc", NULL);
}

static void __exit zs_stat_exit(void)
{
        debugfs_remove_recursive(zs_stat_root);
}

static unsigned long zs_can_compact(struct size_class *class);

static int zs_stats_size_show(struct seq_file *s, void *v)
{
        int i;
        struct zs_pool *pool = s->private;
        struct size_class *class;
        int objs_per_zspage;
        unsigned long class_almost_full, class_almost_empty;
        unsigned long obj_allocated, obj_used, pages_used, freeable;
        unsigned long total_class_almost_full = 0, total_class_almost_empty = 0;
        unsigned long total_objs = 0, total_used_objs = 0, total_pages = 0;
        unsigned long total_freeable = 0;

        seq_printf(s, " %5s %5s %11s %12s %13s %10s %10s %16s %8s\n",
                        "class", "size", "almost_full", "almost_empty",
                        "obj_allocated", "obj_used", "pages_used",
                        "pages_per_zspage", "freeable");

        for (i = 0; i < ZS_SIZE_CLASSES; i++) {
                class = pool->size_class[i];

                if (class->index != i)
                        continue;

                spin_lock(&class->lock);
                class_almost_full = zs_stat_get(class, CLASS_ALMOST_FULL);
                class_almost_empty = zs_stat_get(class, CLASS_ALMOST_EMPTY);
                obj_allocated = zs_stat_get(class, OBJ_ALLOCATED);
                obj_used = zs_stat_get(class, OBJ_USED);
                freeable = zs_can_compact(class);
                spin_unlock(&class->lock);

                objs_per_zspage = class->objs_per_zspage;
                pages_used = obj_allocated / objs_per_zspage *
                                class->pages_per_zspage;

                seq_printf(s, " %5u %5u %11lu %12lu %13lu"
                                " %10lu %10lu %16d %8lu\n",
                        i, class->size, class_almost_full, class_almost_empty,
                        obj_allocated, obj_used, pages_used,
                        class->pages_per_zspage, freeable);

                total_class_almost_full += class_almost_full;
                total_class_almost_empty += class_almost_empty;
                total_objs += obj_allocated;
                total_used_objs += obj_used;
                total_pages += pages_used;
                total_freeable += freeable;
        }

        seq_puts(s, "\n");
        seq_printf(s, " %5s %5s %11lu %12lu %13lu %10lu %10lu %16s %8lu\n",
                        "Total", "", total_class_almost_full,
                        total_class_almost_empty, total_objs,
                        total_used_objs, total_pages, "", total_freeable);

        return 0;
}
DEFINE_SHOW_ATTRIBUTE(zs_stats_size);

static void zs_pool_stat_create(struct zs_pool *pool, const char *name)
{
        if (!zs_stat_root) {
                pr_warn("no root stat dir, not creating <%s> stat dir\n", name);
                return;
        }

        pool->stat_dentry = debugfs_create_dir(name, zs_stat_root);

        debugfs_create_file("classes", S_IFREG | 0444, pool->stat_dentry, pool,
                            &zs_stats_size_fops);
}

static void zs_pool_stat_destroy(struct zs_pool *pool)
{
        debugfs_remove_recursive(pool->stat_dentry);
}

#else /* CONFIG_ZSMALLOC_STAT */
static void __init zs_stat_init(void)
{
}

static void __exit zs_stat_exit(void)
{
}

static inline void zs_pool_stat_create(struct zs_pool *pool, const char *name)
{
}

static inline void zs_pool_stat_destroy(struct zs_pool *pool)
{
}
#endif


/*
 * For each size class, zspages are divided into different groups
 * depending on how "full" they are. This was done so that we could
 * easily find empty or nearly empty zspages when we try to shrink
 * the pool (not yet implemented). This function returns fullness
 * status of the given page.
 */
static enum fullness_group get_fullness_group(struct size_class *class,
                                                struct zspage *zspage)
{
        int inuse, objs_per_zspage;
        enum fullness_group fg;

        inuse = get_zspage_inuse(zspage);
        objs_per_zspage = class->objs_per_zspage;

        if (inuse == 0)
                fg = ZS_EMPTY;
        else if (inuse == objs_per_zspage)
                fg = ZS_FULL;
        else if (inuse <= 3 * objs_per_zspage / fullness_threshold_frac)
                fg = ZS_ALMOST_EMPTY;
        else
                fg = ZS_ALMOST_FULL;

        return fg;
}

/*
 * Each size class maintains various freelists and zspages are assigned
 * to one of these freelists based on the number of live objects they
 * have. This functions inserts the given zspage into the freelist
 * identified by <class, fullness_group>.
 */
static void insert_zspage(struct size_class *class,
                                struct zspage *zspage,
                                enum fullness_group fullness)
{
        struct zspage *head;

        zs_stat_inc(class, fullness, 1);
        head = list_first_entry_or_null(&class->fullness_list[fullness],
                                        struct zspage, list);
        /*
         * We want to see more ZS_FULL pages and less almost empty/full.
         * Put pages with higher ->inuse first.
         */
        if (head) {
                if (get_zspage_inuse(zspage) < get_zspage_inuse(head)) {
                        list_add(&zspage->list, &head->list);
                        return;
                }
        }
        list_add(&zspage->list, &class->fullness_list[fullness]);
}

/*
 * This function removes the given zspage from the freelist identified
 * by <class, fullness_group>.
 */
static void remove_zspage(struct size_class *class,
                                struct zspage *zspage,
                                enum fullness_group fullness)
{
        VM_BUG_ON(list_empty(&class->fullness_list[fullness]));
        VM_BUG_ON(is_zspage_isolated(zspage));

        list_del_init(&zspage->list);
        zs_stat_dec(class, fullness, 1);
}

/*
 * Each size class maintains zspages in different fullness groups depending
 * on the number of live objects they contain. When allocating or freeing
 * objects, the fullness status of the page can change, say, from ALMOST_FULL
 * to ALMOST_EMPTY when freeing an object. This function checks if such
 * a status change has occurred for the given page and accordingly moves the
 * page from the freelist of the old fullness group to that of the new
 * fullness group.
 */
static enum fullness_group fix_fullness_group(struct size_class *class,
                                                struct zspage *zspage)
{
        int class_idx;
        enum fullness_group currfg, newfg;

        get_zspage_mapping(zspage, &class_idx, &currfg);
        newfg = get_fullness_group(class, zspage);
        if (newfg == currfg)
                goto out;

        if (!is_zspage_isolated(zspage)) {
                remove_zspage(class, zspage, currfg);
                insert_zspage(class, zspage, newfg);
        }

        set_zspage_mapping(zspage, class_idx, newfg);

out:
        return newfg;
}

/*
 * We have to decide on how many pages to link together
 * to form a zspage for each size class. This is important
 * to reduce wastage due to unusable space left at end of
 * each zspage which is given as:
 *     wastage = Zp % class_size
 *     usage = Zp - wastage
 * where Zp = zspage size = k * PAGE_SIZE where k = 1, 2, ...
 *
 * For example, for size class of 3/8 * PAGE_SIZE, we should
 * link together 3 PAGE_SIZE sized pages to form a zspage
 * since then we can perfectly fit in 8 such objects.
 */
static int get_pages_per_zspage(int class_size)
{
        int i, max_usedpc = 0;
        /* zspage order which gives maximum used size per KB */
        int max_usedpc_order = 1;

        for (i = 1; i <= ZS_MAX_PAGES_PER_ZSPAGE; i++) {
                int zspage_size;
                int waste, usedpc;

                zspage_size = i * PAGE_SIZE;
                waste = zspage_size % class_size;
                usedpc = (zspage_size - waste) * 100 / zspage_size;

                if (usedpc > max_usedpc) {
                        max_usedpc = usedpc;
                        max_usedpc_order = i;
                }
        }

        return max_usedpc_order;
}

static struct zspage *get_zspage(struct page *page)
{
        struct zspage *zspage = (struct zspage *)page->private;

        BUG_ON(zspage->magic != ZSPAGE_MAGIC);
        return zspage;
}

static struct page *get_next_page(struct page *page)
{
        if (unlikely(PageHugeObject(page)))
                return NULL;

        return page->freelist;
}

/**
 * obj_to_location - get (<page>, <obj_idx>) from encoded object value
 * @obj: the encoded object value
 * @page: page object resides in zspage
 * @obj_idx: object index
 */
static void obj_to_location(unsigned long obj, struct page **page,
                                unsigned int *obj_idx)
{
        obj >>= OBJ_TAG_BITS;
        *page = pfn_to_page(obj >> OBJ_INDEX_BITS);
        *obj_idx = (obj & OBJ_INDEX_MASK);
}

/**
 * location_to_obj - get obj value encoded from (<page>, <obj_idx>)
 * @page: page object resides in zspage
 * @obj_idx: object index
 */
static unsigned long location_to_obj(struct page *page, unsigned int obj_idx)
{
        unsigned long obj;

        obj = page_to_pfn(page) << OBJ_INDEX_BITS;
        obj |= obj_idx & OBJ_INDEX_MASK;
        obj <<= OBJ_TAG_BITS;

        return obj;
}

static unsigned long handle_to_obj(unsigned long handle)
{
        return *(unsigned long *)handle;
}

static unsigned long obj_to_head(struct page *page, void *obj)
{
        if (unlikely(PageHugeObject(page))) {
                VM_BUG_ON_PAGE(!is_first_page(page), page);
                return page->index;
        } else
                return *(unsigned long *)obj;
}

static inline int testpin_tag(unsigned long handle)
{
        return bit_spin_is_locked(HANDLE_PIN_BIT, (unsigned long *)handle);
}

static inline int trypin_tag(unsigned long handle)
{
        return bit_spin_trylock(HANDLE_PIN_BIT, (unsigned long *)handle);
}

static void pin_tag(unsigned long handle)
{
        bit_spin_lock(HANDLE_PIN_BIT, (unsigned long *)handle);
}

static void unpin_tag(unsigned long handle)
{
        bit_spin_unlock(HANDLE_PIN_BIT, (unsigned long *)handle);
}

static void reset_page(struct page *page)
{
        __ClearPageMovable(page);
        ClearPagePrivate(page);
        set_page_private(page, 0);
        page_mapcount_reset(page);
        ClearPageHugeObject(page);
        page->freelist = NULL;
}

static int trylock_zspage(struct zspage *zspage)
{
        struct page *cursor, *fail;

        for (cursor = get_first_page(zspage); cursor != NULL; cursor =
                                        get_next_page(cursor)) {
                if (!trylock_page(cursor)) {
                        fail = cursor;
                        goto unlock;
                }
        }

        return 1;
unlock:
        for (cursor = get_first_page(zspage); cursor != fail; cursor =
                                        get_next_page(cursor))
                unlock_page(cursor);

        return 0;
}

static void __free_zspage(struct zs_pool *pool, struct size_class *class,
                                struct zspage *zspage)
{
        struct page *page, *next;
        enum fullness_group fg;
        unsigned int class_idx;

        get_zspage_mapping(zspage, &class_idx, &fg);

        assert_spin_locked(&class->lock);

        VM_BUG_ON(get_zspage_inuse(zspage));
        VM_BUG_ON(fg != ZS_EMPTY);

        next = page = get_first_page(zspage);
        do {
                VM_BUG_ON_PAGE(!PageLocked(page), page);
                next = get_next_page(page);
                reset_page(page);
                unlock_page(page);
                dec_zone_page_state(page, NR_ZSPAGES);
                put_page(page);
                page = next;
        } while (page != NULL);

        cache_free_zspage(pool, zspage);

        zs_stat_dec(class, OBJ_ALLOCATED, class->objs_per_zspage);
        atomic_long_sub(class->pages_per_zspage,
                                        &pool->pages_allocated);
}

static void free_zspage(struct zs_pool *pool, struct size_class *class,
                                struct zspage *zspage)
{
        VM_BUG_ON(get_zspage_inuse(zspage));
        VM_BUG_ON(list_empty(&zspage->list));

        if (!trylock_zspage(zspage)) {
                kick_deferred_free(pool);
                return;
        }

        remove_zspage(class, zspage, ZS_EMPTY);
        __free_zspage(pool, class, zspage);
}

/* Initialize a newly allocated zspage */
static void init_zspage(struct size_class *class, struct zspage *zspage)
{
        unsigned int freeobj = 1;
        unsigned long off = 0;
        struct page *page = get_first_page(zspage);

        while (page) {
                struct page *next_page;
                struct link_free *link;
                void *vaddr;

                set_first_obj_offset(page, off);

                vaddr = kmap_atomic(page);
                link = (struct link_free *)vaddr + off / sizeof(*link);

                while ((off += class->size) < PAGE_SIZE) {
                        link->next = freeobj++ << OBJ_TAG_BITS;
                        link += class->size / sizeof(*link);
                }

                /*
                 * We now come to the last (full or partial) object on this
                 * page, which must point to the first object on the next
                 * page (if present)
                 */
                next_page = get_next_page(page);
                if (next_page) {
                        link->next = freeobj++ << OBJ_TAG_BITS;
                } else {
                        /*
                         * Reset OBJ_TAG_BITS bit to last link to tell
                         * whether it's allocated object or not.
                         */
                        link->next = -1UL << OBJ_TAG_BITS;
                }
                kunmap_atomic(vaddr);
                page = next_page;
                off %= PAGE_SIZE;
        }

        set_freeobj(zspage, 0);
}

static void create_page_chain(struct size_class *class, struct zspage *zspage,
                                struct page *pages[])
{
        int i;
        struct page *page;
        struct page *prev_page = NULL;
        int nr_pages = class->pages_per_zspage;

        /*
         * Allocate individual pages and link them together as:
         * 1. all pages are linked together using page->freelist
         * 2. each sub-page point to zspage using page->private
         *
         * we set PG_private to identify the first page (i.e. no other sub-page
         * has this flag set).
         */
        for (i = 0; i < nr_pages; i++) {
                page = pages[i];
                set_page_private(page, (unsigned long)zspage);
                page->freelist = NULL;
                if (i == 0) {
                        zspage->first_page = page;
                        SetPagePrivate(page);
                        if (unlikely(class->objs_per_zspage == 1 &&
                                        class->pages_per_zspage == 1))
                                SetPageHugeObject(page);
                } else {
                        prev_page->freelist = page;
                }
                prev_page = page;
        }
}

/*
 * Allocate a zspage for the given size class
 */
static struct zspage *alloc_zspage(struct zs_pool *pool,
                                        struct size_class *class,
                                        gfp_t gfp)
{
        int i;
        struct page *pages[ZS_MAX_PAGES_PER_ZSPAGE];
        struct zspage *zspage = cache_alloc_zspage(pool, gfp);

        if (!zspage)
                return NULL;

        memset(zspage, 0, sizeof(struct zspage));
        zspage->magic = ZSPAGE_MAGIC;
        migrate_lock_init(zspage);

        for (i = 0; i < class->pages_per_zspage; i++) {
                struct page *page;

                page = alloc_page(gfp);
                if (!page) {
                        while (--i >= 0) {
                                dec_zone_page_state(pages[i], NR_ZSPAGES);
                                __free_page(pages[i]);
                        }
                        cache_free_zspage(pool, zspage);
                        return NULL;
                }

                inc_zone_page_state(page, NR_ZSPAGES);
                pages[i] = page;
        }

        create_page_chain(class, zspage, pages);
        init_zspage(class, zspage);

        return zspage;
}

static struct zspage *find_get_zspage(struct size_class *class)
{
        int i;
        struct zspage *zspage;

        for (i = ZS_ALMOST_FULL; i >= ZS_EMPTY; i--) {
                zspage = list_first_entry_or_null(&class->fullness_list[i],
                                struct zspage, list);
                if (zspage)
                        break;
        }

        return zspage;
}

#ifdef CONFIG_PGTABLE_MAPPING
static inline int __zs_cpu_up(struct mapping_area *area)
{
        /*
         * Make sure we don't leak memory if a cpu UP notification
         * and zs_init() race and both call zs_cpu_up() on the same cpu
         */
        if (area->vm)
                return 0;
        area->vm = alloc_vm_area(PAGE_SIZE * 2, NULL);
        if (!area->vm)
                return -ENOMEM;
        return 0;
}

static inline void __zs_cpu_down(struct mapping_area *area)
{
        if (area->vm)
                free_vm_area(area->vm);
        area->vm = NULL;
}

static inline void *__zs_map_object(struct mapping_area *area,
                                struct page *pages[2], int off, int size)
{
        BUG_ON(map_vm_area(area->vm, PAGE_KERNEL, pages));
        area->vm_addr = area->vm->addr;
        return area->vm_addr + off;
}

static inline void __zs_unmap_object(struct mapping_area *area,
                                struct page *pages[2], int off, int size)
{
        unsigned long addr = (unsigned long)area->vm_addr;

        unmap_kernel_range(addr, PAGE_SIZE * 2);
}

#else /* CONFIG_PGTABLE_MAPPING */

static inline int __zs_cpu_up(struct mapping_area *area)
{
        /*
         * Make sure we don't leak memory if a cpu UP notification
         * and zs_init() race and both call zs_cpu_up() on the same cpu
         */
        if (area->vm_buf)
                return 0;
        area->vm_buf = kmalloc(ZS_MAX_ALLOC_SIZE, GFP_KERNEL);
        if (!area->vm_buf)
                return -ENOMEM;
        return 0;
}

static inline void __zs_cpu_down(struct mapping_area *area)
{
        kfree(area->vm_buf);
        area->vm_buf = NULL;
}

static void *__zs_map_object(struct mapping_area *area,
                        struct page *pages[2], int off, int size)
{
        int sizes[2];
        void *addr;
        char *buf = area->vm_buf;

        /* disable page faults to match kmap_atomic() return conditions */
        pagefault_disable();

        /* no read fastpath */
        if (area->vm_mm == ZS_MM_WO)
                goto out;

        sizes[0] = PAGE_SIZE - off;
        sizes[1] = size - sizes[0];

        /* copy object to per-cpu buffer */
        addr = kmap_atomic(pages[0]);
        memcpy(buf, addr + off, sizes[0]);
        kunmap_atomic(addr);
        addr = kmap_atomic(pages[1]);
        memcpy(buf + sizes[0], addr, sizes[1]);
        kunmap_atomic(addr);
out:
        return area->vm_buf;
}

static void __zs_unmap_object(struct mapping_area *area,
                        struct page *pages[2], int off, int size)
{
        int sizes[2];
        void *addr;
        char *buf;

        /* no write fastpath */
        if (area->vm_mm == ZS_MM_RO)
                goto out;

        buf = area->vm_buf;
        buf = buf + ZS_HANDLE_SIZE;
        size -= ZS_HANDLE_SIZE;
        off += ZS_HANDLE_SIZE;

        sizes[0] = PAGE_SIZE - off;
        sizes[1] = size - sizes[0];

        /* copy per-cpu buffer to object */
        addr = kmap_atomic(pages[0]);
        memcpy(addr + off, buf, sizes[0]);
        kunmap_atomic(addr);
        addr = kmap_atomic(pages[1]);
        memcpy(addr, buf + sizes[0], sizes[1]);
        kunmap_atomic(addr);

out:
        /* enable page faults to match kunmap_atomic() return conditions */
        pagefault_enable();
}

#endif /* CONFIG_PGTABLE_MAPPING */

static int zs_cpu_prepare(unsigned int cpu)
{
        struct mapping_area *area;

        area = &per_cpu(zs_map_area, cpu);
        return __zs_cpu_up(area);
}

static int zs_cpu_dead(unsigned int cpu)
{
        struct mapping_area *area;

        area = &per_cpu(zs_map_area, cpu);
        __zs_cpu_down(area);
        return 0;
}

static bool can_merge(struct size_class *prev, int pages_per_zspage,
                                        int objs_per_zspage)
{
        if (prev->pages_per_zspage == pages_per_zspage &&
                prev->objs_per_zspage == objs_per_zspage)
                return true;

        return false;
}

static bool zspage_full(struct size_class *class, struct zspage *zspage)
{
        return get_zspage_inuse(zspage) == class->objs_per_zspage;
}

unsigned long zs_get_total_pages(struct zs_pool *pool)
{
        return atomic_long_read(&pool->pages_allocated);
}
EXPORT_SYMBOL_GPL(zs_get_total_pages);

/**
 * zs_map_object - get address of allocated object from handle.
 * @pool: pool from which the object was allocated
 * @handle: handle returned from zs_malloc
 * @mm: maping mode to use
 *
 * Before using an object allocated from zs_malloc, it must be mapped using
 * this function. When done with the object, it must be unmapped using
 * zs_unmap_object.
 *
 * Only one object can be mapped per cpu at a time. There is no protection
 * against nested mappings.
 *
 * This function returns with preemption and page faults disabled.
 */
void *zs_map_object(struct zs_pool *pool, unsigned long handle,
                        enum zs_mapmode mm)
{
        struct zspage *zspage;
        struct page *page;
        unsigned long obj, off;
        unsigned int obj_idx;

        unsigned int class_idx;
        enum fullness_group fg;
        struct size_class *class;
        struct mapping_area *area;
        struct page *pages[2];
        void *ret;

        /*
         * Because we use per-cpu mapping areas shared among the
         * pools/users, we can't allow mapping in interrupt context
         * because it can corrupt another users mappings.
         */
        BUG_ON(in_interrupt());

        /* From now on, migration cannot move the object */
        pin_tag(handle);

        obj = handle_to_obj(handle);
        obj_to_location(obj, &page, &obj_idx);
        zspage = get_zspage(page);

        /* migration cannot move any subpage in this zspage */
        migrate_read_lock(zspage);

        get_zspage_mapping(zspage, &class_idx, &fg);
        class = pool->size_class[class_idx];
        off = (class->size * obj_idx) & ~PAGE_MASK;

        area = &get_cpu_var(zs_map_area);
        area->vm_mm = mm;
        if (off + class->size <= PAGE_SIZE) {
                /* this object is contained entirely within a page */
                area->vm_addr = kmap_atomic(page);
                ret = area->vm_addr + off;
                goto out;
        }

        /* this object spans two pages */
        pages[0] = page;
        pages[1] = get_next_page(page);
        BUG_ON(!pages[1]);

        ret = __zs_map_object(area, pages, off, class->size);
out:
        if (likely(!PageHugeObject(page)))
                ret += ZS_HANDLE_SIZE;

        return ret;
}
EXPORT_SYMBOL_GPL(zs_map_object);

void zs_unmap_object(struct zs_pool *pool, unsigned long handle)
{
        struct zspage *zspage;
        struct page *page;
        unsigned long obj, off;
        unsigned int obj_idx;

        unsigned int class_idx;
        enum fullness_group fg;
        struct size_class *class;
        struct mapping_area *area;

        obj = handle_to_obj(handle);
        obj_to_location(obj, &page, &obj_idx);
        zspage = get_zspage(page);
        get_zspage_mapping(zspage, &class_idx, &fg);
        class = pool->size_class[class_idx];
        off = (class->size * obj_idx) & ~PAGE_MASK;

        area = this_cpu_ptr(&zs_map_area);
        if (off + class->size <= PAGE_SIZE)
                kunmap_atomic(area->vm_addr);
        else {
                struct page *pages[2];

                pages[0] = page;
                pages[1] = get_next_page(page);
                BUG_ON(!pages[1]);

                __zs_unmap_object(area, pages, off, class->size);
        }
        put_cpu_var(zs_map_area);

        migrate_read_unlock(zspage);
        unpin_tag(handle);
}
EXPORT_SYMBOL_GPL(zs_unmap_object);

/**
 * zs_huge_class_size() - Returns the size (in bytes) of the first huge
 *                        zsmalloc &size_class.
 * @pool: zsmalloc pool to use
 *
 * The function returns the size of the first huge class - any object of equal
 * or bigger size will be stored in zspage consisting of a single physical
 * page.
 *
 * Context: Any context.
 *
 * Return: the size (in bytes) of the first huge zsmalloc &size_class.
 */
size_t zs_huge_class_size(struct zs_pool *pool)
{
        return huge_class_size;
}
EXPORT_SYMBOL_GPL(zs_huge_class_size);

static unsigned long obj_malloc(struct size_class *class,
                                struct zspage *zspage, unsigned long handle)
{
        int i, nr_page, offset;
        unsigned long obj;
        struct link_free *link;

        struct page *m_page;
        unsigned long m_offset;
        void *vaddr;

        handle |= OBJ_ALLOCATED_TAG;
        obj = get_freeobj(zspage);

        offset = obj * class->size;
        nr_page = offset >> PAGE_SHIFT;
        m_offset = offset & ~PAGE_MASK;
        m_page = get_first_page(zspage);

        for (i = 0; i < nr_page; i++)
                m_page = get_next_page(m_page);

        vaddr = kmap_atomic(m_page);
        link = (struct link_free *)vaddr + m_offset / sizeof(*link);
        set_freeobj(zspage, link->next >> OBJ_TAG_BITS);
        if (likely(!PageHugeObject(m_page)))
                /* record handle in the header of allocated chunk */
                link->handle = handle;
        else
                /* record handle to page->index */
                zspage->first_page->index = handle;

        kunmap_atomic(vaddr);
        mod_zspage_inuse(zspage, 1);
        zs_stat_inc(class, OBJ_USED, 1);

        obj = location_to_obj(m_page, obj);

        return obj;
}


/**
 * zs_malloc - Allocate block of given size from pool.
 * @pool: pool to allocate from
 * @size: size of block to allocate
 * @gfp: gfp flags when allocating object
 *
 * On success, handle to the allocated object is returned,
 * otherwise 0.
 * Allocation requests with size > ZS_MAX_ALLOC_SIZE will fail.
 */
unsigned long zs_malloc(struct zs_pool *pool, size_t size, gfp_t gfp)
{
        unsigned long handle, obj;
        struct size_class *class;
        enum fullness_group newfg;
        struct zspage *zspage;

        if (unlikely(!size || size > ZS_MAX_ALLOC_SIZE))
                return 0;

        handle = cache_alloc_handle(pool, gfp);
        if (!handle)
                return 0;

        /* extra space in chunk to keep the handle */
        size += ZS_HANDLE_SIZE;
        class = pool->size_class[get_size_class_index(size)];

        spin_lock(&class->lock);
        zspage = find_get_zspage(class);
        if (likely(zspage)) {
                obj = obj_malloc(class, zspage, handle);
                /* Now move the zspage to another fullness group, if required */
                fix_fullness_group(class, zspage);
                record_obj(handle, obj);
                spin_unlock(&class->lock);

                return handle;
        }

        spin_unlock(&class->lock);

        zspage = alloc_zspage(pool, class, gfp);
        if (!zspage) {
                cache_free_handle(pool, handle);
                return 0;
        }

        spin_lock(&class->lock);
        obj = obj_malloc(class, zspage, handle);
        newfg = get_fullness_group(class, zspage);
        insert_zspage(class, zspage, newfg);
        set_zspage_mapping(zspage, class->index, newfg);
        record_obj(handle, obj);
        atomic_long_add(class->pages_per_zspage,
                                &pool->pages_allocated);
        zs_stat_inc(class, OBJ_ALLOCATED, class->objs_per_zspage);

        /* We completely set up zspage so mark them as movable */
        SetZsPageMovable(pool, zspage);
        spin_unlock(&class->lock);

        return handle;
}
EXPORT_SYMBOL_GPL(zs_malloc);

static void obj_free(struct size_class *class, unsigned long obj)
{
        struct link_free *link;
        struct zspage *zspage;
        struct page *f_page;
        unsigned long f_offset;
        unsigned int f_objidx;
        void *vaddr;

        obj &= ~OBJ_ALLOCATED_TAG;
        obj_to_location(obj, &f_page, &f_objidx);
        f_offset = (class->size * f_objidx) & ~PAGE_MASK;
        zspage = get_zspage(f_page);

        vaddr = kmap_atomic(f_page);

        /* Insert this object in containing zspage's freelist */
        link = (struct link_free *)(vaddr + f_offset);
        link->next = get_freeobj(zspage) << OBJ_TAG_BITS;
        kunmap_atomic(vaddr);
        set_freeobj(zspage, f_objidx);
        mod_zspage_inuse(zspage, -1);
        zs_stat_dec(class, OBJ_USED, 1);
}

void zs_free(struct zs_pool *pool, unsigned long handle)
{
        struct zspage *zspage;
        struct page *f_page;
        unsigned long obj;
        unsigned int f_objidx;
        int class_idx;
        struct size_class *class;
        enum fullness_group fullness;
        bool isolated;

        if (unlikely(!handle))
                return;

        pin_tag(handle);
        obj = handle_to_obj(handle);
        obj_to_location(obj, &f_page, &f_objidx);
        zspage = get_zspage(f_page);

        migrate_read_lock(zspage);

        get_zspage_mapping(zspage, &class_idx, &fullness);
        class = pool->size_class[class_idx];

        spin_lock(&class->lock);
        obj_free(class, obj);
        fullness = fix_fullness_group(class, zspage);
        if (fullness != ZS_EMPTY) {
                migrate_read_unlock(zspage);
                goto out;
        }

        isolated = is_zspage_isolated(zspage);
        migrate_read_unlock(zspage);
        /* If zspage is isolated, zs_page_putback will free the zspage */
        if (likely(!isolated))
                free_zspage(pool, class, zspage);
out:

        spin_unlock(&class->lock);
        unpin_tag(handle);
        cache_free_handle(pool, handle);
}
EXPORT_SYMBOL_GPL(zs_free);

static void zs_object_copy(struct size_class *class, unsigned long dst,
                                unsigned long src)
{
        struct page *s_page, *d_page;
        unsigned int s_objidx, d_objidx;
        unsigned long s_off, d_off;
        void *s_addr, *d_addr;
        int s_size, d_size, size;
        int written = 0;

        s_size = d_size = class->size;

        obj_to_location(src, &s_page, &s_objidx);
        obj_to_location(dst, &d_page, &d_objidx);

        s_off = (class->size * s_objidx) & ~PAGE_MASK;
        d_off = (class->size * d_objidx) & ~PAGE_MASK;

        if (s_off + class->size > PAGE_SIZE)
                s_size = PAGE_SIZE - s_off;

        if (d_off + class->size > PAGE_SIZE)
                d_size = PAGE_SIZE - d_off;

        s_addr = kmap_atomic(s_page);
        d_addr = kmap_atomic(d_page);

        while (1) {
                size = min(s_size, d_size);
                memcpy(d_addr + d_off, s_addr + s_off, size);
                written += size;

                if (written == class->size)
                        break;

                s_off += size;
                s_size -= size;
                d_off += size;
                d_size -= size;

                if (s_off >= PAGE_SIZE) {
                        kunmap_atomic(d_addr);
                        kunmap_atomic(s_addr);
                        s_page = get_next_page(s_page);
                        s_addr = kmap_atomic(s_page);
                        d_addr = kmap_atomic(d_page);
                        s_size = class->size - written;
                        s_off = 0;
                }

                if (d_off >= PAGE_SIZE) {
                        kunmap_atomic(d_addr);
                        d_page = get_next_page(d_page);
                        d_addr = kmap_atomic(d_page);
                        d_size = class->size - written;
                        d_off = 0;
                }
        }

        kunmap_atomic(d_addr);
        kunmap_atomic(s_addr);
}

/*
 * Find alloced object in zspage from index object and
 * return handle.
 */
static unsigned long find_alloced_obj(struct size_class *class,
                                        struct page *page, int *obj_idx)
{
        unsigned long head;
        int offset = 0;
        int index = *obj_idx;
        unsigned long handle = 0;
        void *addr = kmap_atomic(page);

        offset = get_first_obj_offset(page);
        offset += class->size * index;

        while (offset < PAGE_SIZE) {
                head = obj_to_head(page, addr + offset);
                if (head & OBJ_ALLOCATED_TAG) {
                        handle = head & ~OBJ_ALLOCATED_TAG;
                        if (trypin_tag(handle))
                                break;
                        handle = 0;
                }

                offset += class->size;
                index++;
        }

        kunmap_atomic(addr);

        *obj_idx = index;

        return handle;
}

struct zs_compact_control {
        /* Source spage for migration which could be a subpage of zspage */
        struct page *s_page;
        /* Destination page for migration which should be a first page
         * of zspage. */
        struct page *d_page;
         /* Starting object index within @s_page which used for live object
          * in the subpage. */
        int obj_idx;
};

static int migrate_zspage(struct zs_pool *pool, struct size_class *class,
                                struct zs_compact_control *cc)
{
        unsigned long used_obj, free_obj;
        unsigned long handle;
        struct page *s_page = cc->s_page;
        struct page *d_page = cc->d_page;
        int obj_idx = cc->obj_idx;
        int ret = 0;

        while (1) {
                handle = find_alloced_obj(class, s_page, &obj_idx);
                if (!handle) {
                        s_page = get_next_page(s_page);
                        if (!s_page)
                                break;
                        obj_idx = 0;
                        continue;
                }

                /* Stop if there is no more space */
                if (zspage_full(class, get_zspage(d_page))) {
                        unpin_tag(handle);
                        ret = -ENOMEM;
                        break;
                }

                used_obj = handle_to_obj(handle);
                free_obj = obj_malloc(class, get_zspage(d_page), handle);
                zs_object_copy(class, free_obj, used_obj);
                obj_idx++;
                /*
                 * record_obj updates handle's value to free_obj and it will
                 * invalidate lock bit(ie, HANDLE_PIN_BIT) of handle, which
                 * breaks synchronization using pin_tag(e,g, zs_free) so
                 * let's keep the lock bit.
                 */
                free_obj |= BIT(HANDLE_PIN_BIT);
                record_obj(handle, free_obj);
                unpin_tag(handle);
                obj_free(class, used_obj);
        }

        /* Remember last position in this iteration */
        cc->s_page = s_page;
        cc->obj_idx = obj_idx;

        return ret;
}

static struct zspage *isolate_zspage(struct size_class *class, bool source)
{
        int i;
        struct zspage *zspage;
        enum fullness_group fg[2] = {ZS_ALMOST_EMPTY, ZS_ALMOST_FULL};

        if (!source) {
                fg[0] = ZS_ALMOST_FULL;
                fg[1] = ZS_ALMOST_EMPTY;
        }

        for (i = 0; i < 2; i++) {
                zspage = list_first_entry_or_null(&class->fullness_list[fg[i]],
                                                        struct zspage, list);
                if (zspage) {
                        VM_BUG_ON(is_zspage_isolated(zspage));
                        remove_zspage(class, zspage, fg[i]);
                        return zspage;
                }
        }

        return zspage;
}

/*
 * putback_zspage - add @zspage into right class's fullness list
 * @class: destination class
 * @zspage: target page
 *
 * Return @zspage's fullness_group
 */
static enum fullness_group putback_zspage(struct size_class *class,
                        struct zspage *zspage)
{
        enum fullness_group fullness;

        VM_BUG_ON(is_zspage_isolated(zspage));

        fullness = get_fullness_group(class, zspage);
        insert_zspage(class, zspage, fullness);
        set_zspage_mapping(zspage, class->index, fullness);

        return fullness;
}

#ifdef CONFIG_COMPACTION
/*
 * To prevent zspage destroy during migration, zspage freeing should
 * hold locks of all pages in the zspage.
 */
static void lock_zspage(struct zspage *zspage)
{
        struct page *page = get_first_page(zspage);

        do {
                lock_page(page);
        } while ((page = get_next_page(page)) != NULL);
}

static int zs_init_fs_context(struct fs_context *fc)
{
        return init_pseudo(fc, ZSMALLOC_MAGIC) ? 0 : -ENOMEM;
}

static struct file_system_type zsmalloc_fs = {
        .name           = "zsmalloc",
        .init_fs_context = zs_init_fs_context,
        .kill_sb        = kill_anon_super,
};

static int zsmalloc_mount(void)
{
        int ret = 0;

        zsmalloc_mnt = kern_mount(&zsmalloc_fs);
        if (IS_ERR(zsmalloc_mnt))
                ret = PTR_ERR(zsmalloc_mnt);

        return ret;
}

static void zsmalloc_unmount(void)
{
        kern_unmount(zsmalloc_mnt);
}

static void migrate_lock_init(struct zspage *zspage)
{
        rwlock_init(&zspage->lock);
}

static void migrate_read_lock(struct zspage *zspage)
{
        read_lock(&zspage->lock);
}

static void migrate_read_unlock(struct zspage *zspage)
{
        read_unlock(&zspage->lock);
}

static void migrate_write_lock(struct zspage *zspage)
{
        write_lock(&zspage->lock);
}

static void migrate_write_unlock(struct zspage *zspage)
{
        write_unlock(&zspage->lock);
}

/* Number of isolated subpage for *page migration* in this zspage */
static void inc_zspage_isolation(struct zspage *zspage)
{
        zspage->isolated++;
}

static void dec_zspage_isolation(struct zspage *zspage)
{
        zspage->isolated--;
}

static void putback_zspage_deferred(struct zs_pool *pool,
                                    struct size_class *class,
                                    struct zspage *zspage)
{
        enum fullness_group fg;

        fg = putback_zspage(class, zspage);
        if (fg == ZS_EMPTY)
                schedule_work(&pool->free_work);

}

static void replace_sub_page(struct size_class *class, struct zspage *zspage,
                                struct page *newpage, struct page *oldpage)
{
        struct page *page;
        struct page *pages[ZS_MAX_PAGES_PER_ZSPAGE] = {NULL, };
        int idx = 0;

        page = get_first_page(zspage);
        do {
                if (page == oldpage)
                        pages[idx] = newpage;
                else
                        pages[idx] = page;
                idx++;
        } while ((page = get_next_page(page)) != NULL);

        create_page_chain(class, zspage, pages);
        set_first_obj_offset(newpage, get_first_obj_offset(oldpage));
        if (unlikely(PageHugeObject(oldpage)))
                newpage->index = oldpage->index;
        __SetPageMovable(newpage, page_mapping(oldpage));
}

static bool zs_page_isolate(struct page *page, isolate_mode_t mode)
{
        struct zs_pool *pool;
        struct size_class *class;
        int class_idx;
        enum fullness_group fullness;
        struct zspage *zspage;
        struct address_space *mapping;

        /*
         * Page is locked so zspage couldn't be destroyed. For detail, look at
         * lock_zspage in free_zspage.
         */
        VM_BUG_ON_PAGE(!PageMovable(page), page);
        VM_BUG_ON_PAGE(PageIsolated(page), page);

        zspage = get_zspage(page);

        /*
         * Without class lock, fullness could be stale while class_idx is okay
         * because class_idx is constant unless page is freed so we should get
         * fullness again under class lock.
         */
        get_zspage_mapping(zspage, &class_idx, &fullness);
        mapping = page_mapping(page);
        pool = mapping->private_data;
        class = pool->size_class[class_idx];

        spin_lock(&class->lock);
        if (get_zspage_inuse(zspage) == 0) {
                spin_unlock(&class->lock);
                return false;
        }

        /* zspage is isolated for object migration */
        if (list_empty(&zspage->list) && !is_zspage_isolated(zspage)) {
                spin_unlock(&class->lock);
                return false;
        }

        /*
         * If this is first time isolation for the zspage, isolate zspage from
         * size_class to prevent further object allocation from the zspage.
         */
        if (!list_empty(&zspage->list) && !is_zspage_isolated(zspage)) {
                get_zspage_mapping(zspage, &class_idx, &fullness);
                remove_zspage(class, zspage, fullness);
        }

        inc_zspage_isolation(zspage);
        spin_unlock(&class->lock);

        return true;
}

static int zs_page_migrate(struct address_space *mapping, struct page *newpage,
                struct page *page, enum migrate_mode mode)
{
        struct zs_pool *pool;
        struct size_class *class;
        int class_idx;
        enum fullness_group fullness;
        struct zspage *zspage;
        struct page *dummy;
        void *s_addr, *d_addr, *addr;
        int offset, pos;
        unsigned long handle, head;
        unsigned long old_obj, new_obj;
        unsigned int obj_idx;
        int ret = -EAGAIN;

        /*
         * We cannot support the _NO_COPY case here, because copy needs to
         * happen under the zs lock, which does not work with
         * MIGRATE_SYNC_NO_COPY workflow.
         */
        if (mode == MIGRATE_SYNC_NO_COPY)
                return -EINVAL;

        VM_BUG_ON_PAGE(!PageMovable(page), page);
        VM_BUG_ON_PAGE(!PageIsolated(page), page);

        zspage = get_zspage(page);

        /* Concurrent compactor cannot migrate any subpage in zspage */
        migrate_write_lock(zspage);
        get_zspage_mapping(zspage, &class_idx, &fullness);
        pool = mapping->private_data;
        class = pool->size_class[class_idx];
        offset = get_first_obj_offset(page);

        spin_lock(&class->lock);
        if (!get_zspage_inuse(zspage)) {
                /*
                 * Set "offset" to end of the page so that every loops
                 * skips unnecessary object scanning.
                 */
                offset = PAGE_SIZE;
        }

        pos = offset;
        s_addr = kmap_atomic(page);
        while (pos < PAGE_SIZE) {
                head = obj_to_head(page, s_addr + pos);
                if (head & OBJ_ALLOCATED_TAG) {
                        handle = head & ~OBJ_ALLOCATED_TAG;
                        if (!trypin_tag(handle))
                                goto unpin_objects;
                }
                pos += class->size;
        }

        /*
         * Here, any user cannot access all objects in the zspage so let's move.
         */
        d_addr = kmap_atomic(newpage);
        memcpy(d_addr, s_addr, PAGE_SIZE);
        kunmap_atomic(d_addr);

        for (addr = s_addr + offset; addr < s_addr + pos;
                                        addr += class->size) {
                head = obj_to_head(page, addr);
                if (head & OBJ_ALLOCATED_TAG) {
                        handle = head & ~OBJ_ALLOCATED_TAG;
                        if (!testpin_tag(handle))
                                BUG();

                        old_obj = handle_to_obj(handle);
                        obj_to_location(old_obj, &dummy, &obj_idx);
                        new_obj = (unsigned long)location_to_obj(newpage,
                                                                obj_idx);
                        new_obj |= BIT(HANDLE_PIN_BIT);
                        record_obj(handle, new_obj);
                }
        }

        replace_sub_page(class, zspage, newpage, page);
        get_page(newpage);

        dec_zspage_isolation(zspage);

        /*
         * Page migration is done so let's putback isolated zspage to
         * the list if @page is final isolated subpage in the zspage.
         */
        if (!is_zspage_isolated(zspage))
                putback_zspage_deferred(pool, class, zspage);

        reset_page(page);
        put_page(page);
        page = newpage;

        ret = MIGRATEPAGE_SUCCESS;
unpin_objects:
        for (addr = s_addr + offset; addr < s_addr + pos;
                                                addr += class->size) {
                head = obj_to_head(page, addr);
                if (head & OBJ_ALLOCATED_TAG) {
                        handle = head & ~OBJ_ALLOCATED_TAG;
                        if (!testpin_tag(handle))
                                BUG();
                        unpin_tag(handle);
                }
        }
        kunmap_atomic(s_addr);
        spin_unlock(&class->lock);
        migrate_write_unlock(zspage);

        return ret;
}

static void zs_page_putback(struct page *page)
{
        struct zs_pool *pool;
        struct size_class *class;
        int class_idx;
        enum fullness_group fg;
        struct address_space *mapping;
        struct zspage *zspage;

        VM_BUG_ON_PAGE(!PageMovable(page), page);
        VM_BUG_ON_PAGE(!PageIsolated(page), page);

        zspage = get_zspage(page);
        get_zspage_mapping(zspage, &class_idx, &fg);
        mapping = page_mapping(page);
        pool = mapping->private_data;
        class = pool->size_class[class_idx];

        spin_lock(&class->lock);
        dec_zspage_isolation(zspage);
        if (!is_zspage_isolated(zspage)) {
                /*
                 * Due to page_lock, we cannot free zspage immediately
                 * so let's defer.
                 */
                putback_zspage_deferred(pool, class, zspage);
        }

        spin_unlock(&class->lock);
}

static const struct address_space_operations zsmalloc_aops = {
        .isolate_page = zs_page_isolate,
        .migratepage = zs_page_migrate,
        .putback_page = zs_page_putback,
};

static int zs_register_migration(struct zs_pool *pool)
{
        pool->inode = alloc_anon_inode(zsmalloc_mnt->mnt_sb);
        if (IS_ERR(pool->inode)) {
                pool->inode = NULL;
                return 1;
        }

        pool->inode->i_mapping->private_data = pool;
        pool->inode->i_mapping->a_ops = &zsmalloc_aops;
        return 0;
}

static void zs_unregister_migration(struct zs_pool *pool)
{
        flush_work(&pool->free_work);
        iput(pool->inode);
}

/*
 * Caller should hold page_lock of all pages in the zspage
 * In here, we cannot use zspage meta data.
 */
static void async_free_zspage(struct work_struct *work)
{
        int i;
        struct size_class *class;
        unsigned int class_idx;
        enum fullness_group fullness;
        struct zspage *zspage, *tmp;
        LIST_HEAD(free_pages);
        struct zs_pool *pool = container_of(work, struct zs_pool,
                                        free_work);

        for (i = 0; i < ZS_SIZE_CLASSES; i++) {
                class = pool->size_class[i];
                if (class->index != i)
                        continue;

                spin_lock(&class->lock);
                list_splice_init(&class->fullness_list[ZS_EMPTY], &free_pages);
                spin_unlock(&class->lock);
        }


        list_for_each_entry_safe(zspage, tmp, &free_pages, list) {
                list_del(&zspage->list);
                lock_zspage(zspage);

                get_zspage_mapping(zspage, &class_idx, &fullness);
                VM_BUG_ON(fullness != ZS_EMPTY);
                class = pool->size_class[class_idx];
                spin_lock(&class->lock);
                __free_zspage(pool, pool->size_class[class_idx], zspage);
                spin_unlock(&class->lock);
        }
};

static void kick_deferred_free(struct zs_pool *pool)
{
        schedule_work(&pool->free_work);
}

static void init_deferred_free(struct zs_pool *pool)
{
        INIT_WORK(&pool->free_work, async_free_zspage);
}

static void SetZsPageMovable(struct zs_pool *pool, struct zspage *zspage)
{
        struct page *page = get_first_page(zspage);

        do {
                WARN_ON(!trylock_page(page));
                __SetPageMovable(page, pool->inode->i_mapping);
                unlock_page(page);
        } while ((page = get_next_page(page)) != NULL);
}
#endif

/*
 *
 * Based on the number of unused allocated objects calculate
 * and return the number of pages that we can free.
 */
static unsigned long zs_can_compact(struct size_class *class)
{
        unsigned long obj_wasted;
        unsigned long obj_allocated = zs_stat_get(class, OBJ_ALLOCATED);
        unsigned long obj_used = zs_stat_get(class, OBJ_USED);

        if (obj_allocated <= obj_used)
                return 0;

        obj_wasted = obj_allocated - obj_used;
        obj_wasted /= class->objs_per_zspage;

        return obj_wasted * class->pages_per_zspage;
}

static void __zs_compact(struct zs_pool *pool, struct size_class *class)
{
        struct zs_compact_control cc;
        struct zspage *src_zspage;
        struct zspage *dst_zspage = NULL;

        spin_lock(&class->lock);
        while ((src_zspage = isolate_zspage(class, true))) {

                if (!zs_can_compact(class))
                        break;

                cc.obj_idx = 0;
                cc.s_page = get_first_page(src_zspage);

                while ((dst_zspage = isolate_zspage(class, false))) {
                        cc.d_page = get_first_page(dst_zspage);
                        /*
                         * If there is no more space in dst_page, resched
                         * and see if anyone had allocated another zspage.
                         */
                        if (!migrate_zspage(pool, class, &cc))
                                break;

                        putback_zspage(class, dst_zspage);
                }

                /* Stop if we couldn't find slot */
                if (dst_zspage == NULL)
                        break;

                putback_zspage(class, dst_zspage);
                if (putback_zspage(class, src_zspage) == ZS_EMPTY) {
                        free_zspage(pool, class, src_zspage);
                        pool->stats.pages_compacted += class->pages_per_zspage;
                }
                spin_unlock(&class->lock);
                cond_resched();
                spin_lock(&class->lock);
        }

        if (src_zspage)
                putback_zspage(class, src_zspage);

        spin_unlock(&class->lock);
}

unsigned long zs_compact(struct zs_pool *pool)
{
        int i;
        struct size_class *class;

        for (i = ZS_SIZE_CLASSES - 1; i >= 0; i--) {
                class = pool->size_class[i];
                if (!class)
                        continue;
                if (class->index != i)
                        continue;
                __zs_compact(pool, class);
        }

        return pool->stats.pages_compacted;
}
EXPORT_SYMBOL_GPL(zs_compact);

void zs_pool_stats(struct zs_pool *pool, struct zs_pool_stats *stats)
{
        memcpy(stats, &pool->stats, sizeof(struct zs_pool_stats));
}
EXPORT_SYMBOL_GPL(zs_pool_stats);

static unsigned long zs_shrinker_scan(struct shrinker *shrinker,
                struct shrink_control *sc)
{
        unsigned long pages_freed;
        struct zs_pool *pool = container_of(shrinker, struct zs_pool,
                        shrinker);

        pages_freed = pool->stats.pages_compacted;
        /*
         * Compact classes and calculate compaction delta.
         * Can run concurrently with a manually triggered
         * (by user) compaction.
         */
        pages_freed = zs_compact(pool) - pages_freed;

        return pages_freed ? pages_freed : SHRINK_STOP;
}

static unsigned long zs_shrinker_count(struct shrinker *shrinker,
                struct shrink_control *sc)
{
        int i;
        struct size_class *class;
        unsigned long pages_to_free = 0;
        struct zs_pool *pool = container_of(shrinker, struct zs_pool,
                        shrinker);

        for (i = ZS_SIZE_CLASSES - 1; i >= 0; i--) {
                class = pool->size_class[i];
                if (!class)
                        continue;
                if (class->index != i)
                        continue;

                pages_to_free += zs_can_compact(class);
        }

        return pages_to_free;
}

static void zs_unregister_shrinker(struct zs_pool *pool)
{
        unregister_shrinker(&pool->shrinker);
}

static int zs_register_shrinker(struct zs_pool *pool)
{
        pool->shrinker.scan_objects = zs_shrinker_scan;
        pool->shrinker.count_objects = zs_shrinker_count;
        pool->shrinker.batch = 0;
        pool->shrinker.seeks = DEFAULT_SEEKS;

        return register_shrinker(&pool->shrinker);
}

/**
 * zs_create_pool - Creates an allocation pool to work from.
 * @name: pool name to be created
 *
 * This function must be called before anything when using
 * the zsmalloc allocator.
 *
 * On success, a pointer to the newly created pool is returned,
 * otherwise NULL.
 */
struct zs_pool *zs_create_pool(const char *name)
{
        int i;
        struct zs_pool *pool;
        struct size_class *prev_class = NULL;

        pool = kzalloc(sizeof(*pool), GFP_KERNEL);
        if (!pool)
                return NULL;

        init_deferred_free(pool);

        pool->name = kstrdup(name, GFP_KERNEL);
        if (!pool->name)
                goto err;

        if (create_cache(pool))
                goto err;

        /*
         * Iterate reversely, because, size of size_class that we want to use
         * for merging should be larger or equal to current size.
         */
        for (i = ZS_SIZE_CLASSES - 1; i >= 0; i--) {
                int size;
                int pages_per_zspage;
                int objs_per_zspage;
                struct size_class *class;
                int fullness = 0;

                size = ZS_MIN_ALLOC_SIZE + i * ZS_SIZE_CLASS_DELTA;
                if (size > ZS_MAX_ALLOC_SIZE)
                        size = ZS_MAX_ALLOC_SIZE;
                pages_per_zspage = get_pages_per_zspage(size);
                objs_per_zspage = pages_per_zspage * PAGE_SIZE / size;

                /*
                 * We iterate from biggest down to smallest classes,
                 * so huge_class_size holds the size of the first huge
                 * class. Any object bigger than or equal to that will
                 * endup in the huge class.
                 */
                if (pages_per_zspage != 1 && objs_per_zspage != 1 &&
                                !huge_class_size) {
                        huge_class_size = size;
                        /*
                         * The object uses ZS_HANDLE_SIZE bytes to store the
                         * handle. We need to subtract it, because zs_malloc()
                         * unconditionally adds handle size before it performs
                         * size class search - so object may be smaller than
                         * huge class size, yet it still can end up in the huge
                         * class because it grows by ZS_HANDLE_SIZE extra bytes
                         * right before class lookup.
                         */
                        huge_class_size -= (ZS_HANDLE_SIZE - 1);
                }

                /*
                 * size_class is used for normal zsmalloc operation such
                 * as alloc/free for that size. Although it is natural that we
                 * have one size_class for each size, there is a chance that we
                 * can get more memory utilization if we use one size_class for
                 * many different sizes whose size_class have same
                 * characteristics. So, we makes size_class point to
                 * previous size_class if possible.
                 */
                if (prev_class) {
                        if (can_merge(prev_class, pages_per_zspage, objs_per_zspage)) {
                                pool->size_class[i] = prev_class;
                                continue;
                        }
                }

                class = kzalloc(sizeof(struct size_class), GFP_KERNEL);
                if (!class)
                        goto err;

                class->size = size;
                class->index = i;
                class->pages_per_zspage = pages_per_zspage;
                class->objs_per_zspage = objs_per_zspage;
                spin_lock_init(&class->lock);
                pool->size_class[i] = class;
                for (fullness = ZS_EMPTY; fullness < NR_ZS_FULLNESS;
                                                        fullness++)
                        INIT_LIST_HEAD(&class->fullness_list[fullness]);

                prev_class = class;
        }

        /* debug only, don't abort if it fails */
        zs_pool_stat_create(pool, name);

        if (zs_register_migration(pool))
                goto err;

        /*
         * Not critical since shrinker is only used to trigger internal
         * defragmentation of the pool which is pretty optional thing.  If
         * registration fails we still can use the pool normally and user can
         * trigger compaction manually. Thus, ignore return code.
         */
        zs_register_shrinker(pool);

        return pool;

err:
        zs_destroy_pool(pool);
        return NULL;
}
EXPORT_SYMBOL_GPL(zs_create_pool);

void zs_destroy_pool(struct zs_pool *pool)
{
        int i;

        zs_unregister_shrinker(pool);
        zs_unregister_migration(pool);
        zs_pool_stat_destroy(pool);

        for (i = 0; i < ZS_SIZE_CLASSES; i++) {
                int fg;
                struct size_class *class = pool->size_class[i];

                if (!class)
                        continue;

                if (class->index != i)
                        continue;

                for (fg = ZS_EMPTY; fg < NR_ZS_FULLNESS; fg++) {
                        if (!list_empty(&class->fullness_list[fg])) {
                                pr_info("Freeing non-empty class with size %db, fullness group %d\n",
                                        class->size, fg);
                        }
                }
                kfree(class);
        }

        destroy_cache(pool);
        kfree(pool->name);
        kfree(pool);
}
EXPORT_SYMBOL_GPL(zs_destroy_pool);

static int __init zs_init(void)
{
        int ret;

        ret = zsmalloc_mount();
        if (ret)
                goto out;

        ret = cpuhp_setup_state(CPUHP_MM_ZS_PREPARE, "mm/zsmalloc:prepare",
                                zs_cpu_prepare, zs_cpu_dead);
        if (ret)
                goto hp_setup_fail;

#ifdef CONFIG_ZPOOL
        zpool_register_driver(&zs_zpool_driver);
#endif

        zs_stat_init();

        return 0;

hp_setup_fail:
        zsmalloc_unmount();
out:
        return ret;
}

static void __exit zs_exit(void)
{
#ifdef CONFIG_ZPOOL
        zpool_unregister_driver(&zs_zpool_driver);
#endif
        zsmalloc_unmount();
        cpuhp_remove_state(CPUHP_MM_ZS_PREPARE);

        zs_stat_exit();
}

module_init(zs_init);
module_exit(zs_exit);

MODULE_LICENSE("Dual BSD/GPL");
MODULE_AUTHOR("Nitin Gupta <ngupta@vflare.org>");