doc: Update doc about journalling layer

[sfrench/cifs-2.6.git] / fs / jbd / transaction.c

/*
 * linux/fs/jbd/transaction.c
 *
 * Written by Stephen C. Tweedie <sct@redhat.com>, 1998
 *
 * Copyright 1998 Red Hat corp --- All Rights Reserved
 *
 * This file is part of the Linux kernel and is made available under
 * the terms of the GNU General Public License, version 2, or at your
 * option, any later version, incorporated herein by reference.
 *
 * Generic filesystem transaction handling code; part of the ext2fs
 * journaling system.
 *
 * This file manages transactions (compound commits managed by the
 * journaling code) and handles (individual atomic operations by the
 * filesystem).
 */

#include <linux/time.h>
#include <linux/fs.h>
#include <linux/jbd.h>
#include <linux/errno.h>
#include <linux/slab.h>
#include <linux/timer.h>
#include <linux/mm.h>
#include <linux/highmem.h>
#include <linux/hrtimer.h>

static void __journal_temp_unlink_buffer(struct journal_head *jh);

/*
 * get_transaction: obtain a new transaction_t object.
 *
 * Simply allocate and initialise a new transaction.  Create it in
 * RUNNING state and add it to the current journal (which should not
 * have an existing running transaction: we only make a new transaction
 * once we have started to commit the old one).
 *
 * Preconditions:
 *      The journal MUST be locked.  We don't perform atomic mallocs on the
 *      new transaction and we can't block without protecting against other
 *      processes trying to touch the journal while it is in transition.
 *
 * Called under j_state_lock
 */

static transaction_t *
get_transaction(journal_t *journal, transaction_t *transaction)
{
        transaction->t_journal = journal;
        transaction->t_state = T_RUNNING;
        transaction->t_start_time = ktime_get();
        transaction->t_tid = journal->j_transaction_sequence++;
        transaction->t_expires = jiffies + journal->j_commit_interval;
        spin_lock_init(&transaction->t_handle_lock);

        /* Set up the commit timer for the new transaction. */
        journal->j_commit_timer.expires =
                                round_jiffies_up(transaction->t_expires);
        add_timer(&journal->j_commit_timer);

        J_ASSERT(journal->j_running_transaction == NULL);
        journal->j_running_transaction = transaction;

        return transaction;
}

/*
 * Handle management.
 *
 * A handle_t is an object which represents a single atomic update to a
 * filesystem, and which tracks all of the modifications which form part
 * of that one update.
 */

/*
 * start_this_handle: Given a handle, deal with any locking or stalling
 * needed to make sure that there is enough journal space for the handle
 * to begin.  Attach the handle to a transaction and set up the
 * transaction's buffer credits.
 */

static int start_this_handle(journal_t *journal, handle_t *handle)
{
        transaction_t *transaction;
        int needed;
        int nblocks = handle->h_buffer_credits;
        transaction_t *new_transaction = NULL;
        int ret = 0;

        if (nblocks > journal->j_max_transaction_buffers) {
                printk(KERN_ERR "JBD: %s wants too many credits (%d > %d)\n",
                       current->comm, nblocks,
                       journal->j_max_transaction_buffers);
                ret = -ENOSPC;
                goto out;
        }

alloc_transaction:
        if (!journal->j_running_transaction) {
                new_transaction = kzalloc(sizeof(*new_transaction),
                                                GFP_NOFS|__GFP_NOFAIL);
                if (!new_transaction) {
                        ret = -ENOMEM;
                        goto out;
                }
        }

        jbd_debug(3, "New handle %p going live.\n", handle);

repeat:

        /*
         * We need to hold j_state_lock until t_updates has been incremented,
         * for proper journal barrier handling
         */
        spin_lock(&journal->j_state_lock);
repeat_locked:
        if (is_journal_aborted(journal) ||
            (journal->j_errno != 0 && !(journal->j_flags & JFS_ACK_ERR))) {
                spin_unlock(&journal->j_state_lock);
                ret = -EROFS;
                goto out;
        }

        /* Wait on the journal's transaction barrier if necessary */
        if (journal->j_barrier_count) {
                spin_unlock(&journal->j_state_lock);
                wait_event(journal->j_wait_transaction_locked,
                                journal->j_barrier_count == 0);
                goto repeat;
        }

        if (!journal->j_running_transaction) {
                if (!new_transaction) {
                        spin_unlock(&journal->j_state_lock);
                        goto alloc_transaction;
                }
                get_transaction(journal, new_transaction);
                new_transaction = NULL;
        }

        transaction = journal->j_running_transaction;

        /*
         * If the current transaction is locked down for commit, wait for the
         * lock to be released.
         */
        if (transaction->t_state == T_LOCKED) {
                DEFINE_WAIT(wait);

                prepare_to_wait(&journal->j_wait_transaction_locked,
                                        &wait, TASK_UNINTERRUPTIBLE);
                spin_unlock(&journal->j_state_lock);
                schedule();
                finish_wait(&journal->j_wait_transaction_locked, &wait);
                goto repeat;
        }

        /*
         * If there is not enough space left in the log to write all potential
         * buffers requested by this operation, we need to stall pending a log
         * checkpoint to free some more log space.
         */
        spin_lock(&transaction->t_handle_lock);
        needed = transaction->t_outstanding_credits + nblocks;

        if (needed > journal->j_max_transaction_buffers) {
                /*
                 * If the current transaction is already too large, then start
                 * to commit it: we can then go back and attach this handle to
                 * a new transaction.
                 */
                DEFINE_WAIT(wait);

                jbd_debug(2, "Handle %p starting new commit...\n", handle);
                spin_unlock(&transaction->t_handle_lock);
                prepare_to_wait(&journal->j_wait_transaction_locked, &wait,
                                TASK_UNINTERRUPTIBLE);
                __log_start_commit(journal, transaction->t_tid);
                spin_unlock(&journal->j_state_lock);
                schedule();
                finish_wait(&journal->j_wait_transaction_locked, &wait);
                goto repeat;
        }

        /*
         * The commit code assumes that it can get enough log space
         * without forcing a checkpoint.  This is *critical* for
         * correctness: a checkpoint of a buffer which is also
         * associated with a committing transaction creates a deadlock,
         * so commit simply cannot force through checkpoints.
         *
         * We must therefore ensure the necessary space in the journal
         * *before* starting to dirty potentially checkpointed buffers
         * in the new transaction.
         *
         * The worst part is, any transaction currently committing can
         * reduce the free space arbitrarily.  Be careful to account for
         * those buffers when checkpointing.
         */

        /*
         * @@@ AKPM: This seems rather over-defensive.  We're giving commit
         * a _lot_ of headroom: 1/4 of the journal plus the size of
         * the committing transaction.  Really, we only need to give it
         * committing_transaction->t_outstanding_credits plus "enough" for
         * the log control blocks.
         * Also, this test is inconsistent with the matching one in
         * journal_extend().
         */
        if (__log_space_left(journal) < jbd_space_needed(journal)) {
                jbd_debug(2, "Handle %p waiting for checkpoint...\n", handle);
                spin_unlock(&transaction->t_handle_lock);
                __log_wait_for_space(journal);
                goto repeat_locked;
        }

        /* OK, account for the buffers that this operation expects to
         * use and add the handle to the running transaction. */

        handle->h_transaction = transaction;
        transaction->t_outstanding_credits += nblocks;
        transaction->t_updates++;
        transaction->t_handle_count++;
        jbd_debug(4, "Handle %p given %d credits (total %d, free %d)\n",
                  handle, nblocks, transaction->t_outstanding_credits,
                  __log_space_left(journal));
        spin_unlock(&transaction->t_handle_lock);
        spin_unlock(&journal->j_state_lock);

        lock_map_acquire(&handle->h_lockdep_map);
out:
        if (unlikely(new_transaction))          /* It's usually NULL */
                kfree(new_transaction);
        return ret;
}

static struct lock_class_key jbd_handle_key;

/* Allocate a new handle.  This should probably be in a slab... */
static handle_t *new_handle(int nblocks)
{
        handle_t *handle = jbd_alloc_handle(GFP_NOFS);
        if (!handle)
                return NULL;
        handle->h_buffer_credits = nblocks;
        handle->h_ref = 1;

        lockdep_init_map(&handle->h_lockdep_map, "jbd_handle", &jbd_handle_key, 0);

        return handle;
}

/**
 * handle_t *journal_start() - Obtain a new handle.
 * @journal: Journal to start transaction on.
 * @nblocks: number of block buffer we might modify
 *
 * We make sure that the transaction can guarantee at least nblocks of
 * modified buffers in the log.  We block until the log can guarantee
 * that much space.
 *
 * This function is visible to journal users (like ext3fs), so is not
 * called with the journal already locked.
 *
 * Return a pointer to a newly allocated handle, or an ERR_PTR() value
 * on failure.
 */
handle_t *journal_start(journal_t *journal, int nblocks)
{
        handle_t *handle = journal_current_handle();
        int err;

        if (!journal)
                return ERR_PTR(-EROFS);

        if (handle) {
                J_ASSERT(handle->h_transaction->t_journal == journal);
                handle->h_ref++;
                return handle;
        }

        handle = new_handle(nblocks);
        if (!handle)
                return ERR_PTR(-ENOMEM);

        current->journal_info = handle;

        err = start_this_handle(journal, handle);
        if (err < 0) {
                jbd_free_handle(handle);
                current->journal_info = NULL;
                handle = ERR_PTR(err);
        }
        return handle;
}

/**
 * int journal_extend() - extend buffer credits.
 * @handle:  handle to 'extend'
 * @nblocks: nr blocks to try to extend by.
 *
 * Some transactions, such as large extends and truncates, can be done
 * atomically all at once or in several stages.  The operation requests
 * a credit for a number of buffer modications in advance, but can
 * extend its credit if it needs more.
 *
 * journal_extend tries to give the running handle more buffer credits.
 * It does not guarantee that allocation - this is a best-effort only.
 * The calling process MUST be able to deal cleanly with a failure to
 * extend here.
 *
 * Return 0 on success, non-zero on failure.
 *
 * return code < 0 implies an error
 * return code > 0 implies normal transaction-full status.
 */
int journal_extend(handle_t *handle, int nblocks)
{
        transaction_t *transaction = handle->h_transaction;
        journal_t *journal = transaction->t_journal;
        int result;
        int wanted;

        result = -EIO;
        if (is_handle_aborted(handle))
                goto out;

        result = 1;

        spin_lock(&journal->j_state_lock);

        /* Don't extend a locked-down transaction! */
        if (handle->h_transaction->t_state != T_RUNNING) {
                jbd_debug(3, "denied handle %p %d blocks: "
                          "transaction not running\n", handle, nblocks);
                goto error_out;
        }

        spin_lock(&transaction->t_handle_lock);
        wanted = transaction->t_outstanding_credits + nblocks;

        if (wanted > journal->j_max_transaction_buffers) {
                jbd_debug(3, "denied handle %p %d blocks: "
                          "transaction too large\n", handle, nblocks);
                goto unlock;
        }

        if (wanted > __log_space_left(journal)) {
                jbd_debug(3, "denied handle %p %d blocks: "
                          "insufficient log space\n", handle, nblocks);
                goto unlock;
        }

        handle->h_buffer_credits += nblocks;
        transaction->t_outstanding_credits += nblocks;
        result = 0;

        jbd_debug(3, "extended handle %p by %d\n", handle, nblocks);
unlock:
        spin_unlock(&transaction->t_handle_lock);
error_out:
        spin_unlock(&journal->j_state_lock);
out:
        return result;
}


/**
 * int journal_restart() - restart a handle.
 * @handle:  handle to restart
 * @nblocks: nr credits requested
 *
 * Restart a handle for a multi-transaction filesystem
 * operation.
 *
 * If the journal_extend() call above fails to grant new buffer credits
 * to a running handle, a call to journal_restart will commit the
 * handle's transaction so far and reattach the handle to a new
 * transaction capabable of guaranteeing the requested number of
 * credits.
 */

int journal_restart(handle_t *handle, int nblocks)
{
        transaction_t *transaction = handle->h_transaction;
        journal_t *journal = transaction->t_journal;
        int ret;

        /* If we've had an abort of any type, don't even think about
         * actually doing the restart! */
        if (is_handle_aborted(handle))
                return 0;

        /*
         * First unlink the handle from its current transaction, and start the
         * commit on that.
         */
        J_ASSERT(transaction->t_updates > 0);
        J_ASSERT(journal_current_handle() == handle);

        spin_lock(&journal->j_state_lock);
        spin_lock(&transaction->t_handle_lock);
        transaction->t_outstanding_credits -= handle->h_buffer_credits;
        transaction->t_updates--;

        if (!transaction->t_updates)
                wake_up(&journal->j_wait_updates);
        spin_unlock(&transaction->t_handle_lock);

        jbd_debug(2, "restarting handle %p\n", handle);
        __log_start_commit(journal, transaction->t_tid);
        spin_unlock(&journal->j_state_lock);

        lock_map_release(&handle->h_lockdep_map);
        handle->h_buffer_credits = nblocks;
        ret = start_this_handle(journal, handle);
        return ret;
}


/**
 * void journal_lock_updates () - establish a transaction barrier.
 * @journal:  Journal to establish a barrier on.
 *
 * This locks out any further updates from being started, and blocks until all
 * existing updates have completed, returning only once the journal is in a
 * quiescent state with no updates running.
 *
 * We do not use simple mutex for synchronization as there are syscalls which
 * want to return with filesystem locked and that trips up lockdep. Also
 * hibernate needs to lock filesystem but locked mutex then blocks hibernation.
 * Since locking filesystem is rare operation, we use simple counter and
 * waitqueue for locking.
 */
void journal_lock_updates(journal_t *journal)
{
        DEFINE_WAIT(wait);

wait:
        /* Wait for previous locked operation to finish */
        wait_event(journal->j_wait_transaction_locked,
                   journal->j_barrier_count == 0);

        spin_lock(&journal->j_state_lock);
        /*
         * Check reliably under the lock whether we are the ones winning the race
         * and locking the journal
         */
        if (journal->j_barrier_count > 0) {
                spin_unlock(&journal->j_state_lock);
                goto wait;
        }
        ++journal->j_barrier_count;

        /* Wait until there are no running updates */
        while (1) {
                transaction_t *transaction = journal->j_running_transaction;

                if (!transaction)
                        break;

                spin_lock(&transaction->t_handle_lock);
                if (!transaction->t_updates) {
                        spin_unlock(&transaction->t_handle_lock);
                        break;
                }
                prepare_to_wait(&journal->j_wait_updates, &wait,
                                TASK_UNINTERRUPTIBLE);
                spin_unlock(&transaction->t_handle_lock);
                spin_unlock(&journal->j_state_lock);
                schedule();
                finish_wait(&journal->j_wait_updates, &wait);
                spin_lock(&journal->j_state_lock);
        }
        spin_unlock(&journal->j_state_lock);
}

/**
 * void journal_unlock_updates (journal_t* journal) - release barrier
 * @journal:  Journal to release the barrier on.
 *
 * Release a transaction barrier obtained with journal_lock_updates().
 */
void journal_unlock_updates (journal_t *journal)
{
        J_ASSERT(journal->j_barrier_count != 0);

        spin_lock(&journal->j_state_lock);
        --journal->j_barrier_count;
        spin_unlock(&journal->j_state_lock);
        wake_up(&journal->j_wait_transaction_locked);
}

static void warn_dirty_buffer(struct buffer_head *bh)
{
        char b[BDEVNAME_SIZE];

        printk(KERN_WARNING
               "JBD: Spotted dirty metadata buffer (dev = %s, blocknr = %llu). "
               "There's a risk of filesystem corruption in case of system "
               "crash.\n",
               bdevname(bh->b_bdev, b), (unsigned long long)bh->b_blocknr);
}

/*
 * If the buffer is already part of the current transaction, then there
 * is nothing we need to do.  If it is already part of a prior
 * transaction which we are still committing to disk, then we need to
 * make sure that we do not overwrite the old copy: we do copy-out to
 * preserve the copy going to disk.  We also account the buffer against
 * the handle's metadata buffer credits (unless the buffer is already
 * part of the transaction, that is).
 *
 */
static int
do_get_write_access(handle_t *handle, struct journal_head *jh,
                        int force_copy)
{
        struct buffer_head *bh;
        transaction_t *transaction;
        journal_t *journal;
        int error;
        char *frozen_buffer = NULL;
        int need_copy = 0;

        if (is_handle_aborted(handle))
                return -EROFS;

        transaction = handle->h_transaction;
        journal = transaction->t_journal;

        jbd_debug(5, "journal_head %p, force_copy %d\n", jh, force_copy);

        JBUFFER_TRACE(jh, "entry");
repeat:
        bh = jh2bh(jh);

        /* @@@ Need to check for errors here at some point. */

        lock_buffer(bh);
        jbd_lock_bh_state(bh);

        /* We now hold the buffer lock so it is safe to query the buffer
         * state.  Is the buffer dirty?
         *
         * If so, there are two possibilities.  The buffer may be
         * non-journaled, and undergoing a quite legitimate writeback.
         * Otherwise, it is journaled, and we don't expect dirty buffers
         * in that state (the buffers should be marked JBD_Dirty
         * instead.)  So either the IO is being done under our own
         * control and this is a bug, or it's a third party IO such as
         * dump(8) (which may leave the buffer scheduled for read ---
         * ie. locked but not dirty) or tune2fs (which may actually have
         * the buffer dirtied, ugh.)  */

        if (buffer_dirty(bh)) {
                /*
                 * First question: is this buffer already part of the current
                 * transaction or the existing committing transaction?
                 */
                if (jh->b_transaction) {
                        J_ASSERT_JH(jh,
                                jh->b_transaction == transaction ||
                                jh->b_transaction ==
                                        journal->j_committing_transaction);
                        if (jh->b_next_transaction)
                                J_ASSERT_JH(jh, jh->b_next_transaction ==
                                                        transaction);
                        warn_dirty_buffer(bh);
                }
                /*
                 * In any case we need to clean the dirty flag and we must
                 * do it under the buffer lock to be sure we don't race
                 * with running write-out.
                 */
                JBUFFER_TRACE(jh, "Journalling dirty buffer");
                clear_buffer_dirty(bh);
                set_buffer_jbddirty(bh);
        }

        unlock_buffer(bh);

        error = -EROFS;
        if (is_handle_aborted(handle)) {
                jbd_unlock_bh_state(bh);
                goto out;
        }
        error = 0;

        /*
         * The buffer is already part of this transaction if b_transaction or
         * b_next_transaction points to it
         */
        if (jh->b_transaction == transaction ||
            jh->b_next_transaction == transaction)
                goto done;

        /*
         * this is the first time this transaction is touching this buffer,
         * reset the modified flag
         */
        jh->b_modified = 0;

        /*
         * If there is already a copy-out version of this buffer, then we don't
         * need to make another one
         */
        if (jh->b_frozen_data) {
                JBUFFER_TRACE(jh, "has frozen data");
                J_ASSERT_JH(jh, jh->b_next_transaction == NULL);
                jh->b_next_transaction = transaction;
                goto done;
        }

        /* Is there data here we need to preserve? */

        if (jh->b_transaction && jh->b_transaction != transaction) {
                JBUFFER_TRACE(jh, "owned by older transaction");
                J_ASSERT_JH(jh, jh->b_next_transaction == NULL);
                J_ASSERT_JH(jh, jh->b_transaction ==
                                        journal->j_committing_transaction);

                /* There is one case we have to be very careful about.
                 * If the committing transaction is currently writing
                 * this buffer out to disk and has NOT made a copy-out,
                 * then we cannot modify the buffer contents at all
                 * right now.  The essence of copy-out is that it is the
                 * extra copy, not the primary copy, which gets
                 * journaled.  If the primary copy is already going to
                 * disk then we cannot do copy-out here. */

                if (jh->b_jlist == BJ_Shadow) {
                        DEFINE_WAIT_BIT(wait, &bh->b_state, BH_Unshadow);
                        wait_queue_head_t *wqh;

                        wqh = bit_waitqueue(&bh->b_state, BH_Unshadow);

                        JBUFFER_TRACE(jh, "on shadow: sleep");
                        jbd_unlock_bh_state(bh);
                        /* commit wakes up all shadow buffers after IO */
                        for ( ; ; ) {
                                prepare_to_wait(wqh, &wait.wait,
                                                TASK_UNINTERRUPTIBLE);
                                if (jh->b_jlist != BJ_Shadow)
                                        break;
                                schedule();
                        }
                        finish_wait(wqh, &wait.wait);
                        goto repeat;
                }

                /* Only do the copy if the currently-owning transaction
                 * still needs it.  If it is on the Forget list, the
                 * committing transaction is past that stage.  The
                 * buffer had better remain locked during the kmalloc,
                 * but that should be true --- we hold the journal lock
                 * still and the buffer is already on the BUF_JOURNAL
                 * list so won't be flushed.
                 *
                 * Subtle point, though: if this is a get_undo_access,
                 * then we will be relying on the frozen_data to contain
                 * the new value of the committed_data record after the
                 * transaction, so we HAVE to force the frozen_data copy
                 * in that case. */

                if (jh->b_jlist != BJ_Forget || force_copy) {
                        JBUFFER_TRACE(jh, "generate frozen data");
                        if (!frozen_buffer) {
                                JBUFFER_TRACE(jh, "allocate memory for buffer");
                                jbd_unlock_bh_state(bh);
                                frozen_buffer =
                                        jbd_alloc(jh2bh(jh)->b_size,
                                                         GFP_NOFS);
                                if (!frozen_buffer) {
                                        printk(KERN_ERR
                                               "%s: OOM for frozen_buffer\n",
                                               __func__);
                                        JBUFFER_TRACE(jh, "oom!");
                                        error = -ENOMEM;
                                        jbd_lock_bh_state(bh);
                                        goto done;
                                }
                                goto repeat;
                        }
                        jh->b_frozen_data = frozen_buffer;
                        frozen_buffer = NULL;
                        need_copy = 1;
                }
                jh->b_next_transaction = transaction;
        }


        /*
         * Finally, if the buffer is not journaled right now, we need to make
         * sure it doesn't get written to disk before the caller actually
         * commits the new data
         */
        if (!jh->b_transaction) {
                JBUFFER_TRACE(jh, "no transaction");
                J_ASSERT_JH(jh, !jh->b_next_transaction);
                JBUFFER_TRACE(jh, "file as BJ_Reserved");
                spin_lock(&journal->j_list_lock);
                __journal_file_buffer(jh, transaction, BJ_Reserved);
                spin_unlock(&journal->j_list_lock);
        }

done:
        if (need_copy) {
                struct page *page;
                int offset;
                char *source;

                J_EXPECT_JH(jh, buffer_uptodate(jh2bh(jh)),
                            "Possible IO failure.\n");
                page = jh2bh(jh)->b_page;
                offset = offset_in_page(jh2bh(jh)->b_data);
                source = kmap_atomic(page);
                memcpy(jh->b_frozen_data, source+offset, jh2bh(jh)->b_size);
                kunmap_atomic(source);
        }
        jbd_unlock_bh_state(bh);

        /*
         * If we are about to journal a buffer, then any revoke pending on it is
         * no longer valid
         */
        journal_cancel_revoke(handle, jh);

out:
        if (unlikely(frozen_buffer))    /* It's usually NULL */
                jbd_free(frozen_buffer, bh->b_size);

        JBUFFER_TRACE(jh, "exit");
        return error;
}

/**
 * int journal_get_write_access() - notify intent to modify a buffer for metadata (not data) update.
 * @handle: transaction to add buffer modifications to
 * @bh:     bh to be used for metadata writes
 *
 * Returns an error code or 0 on success.
 *
 * In full data journalling mode the buffer may be of type BJ_AsyncData,
 * because we're write()ing a buffer which is also part of a shared mapping.
 */

int journal_get_write_access(handle_t *handle, struct buffer_head *bh)
{
        struct journal_head *jh = journal_add_journal_head(bh);
        int rc;

        /* We do not want to get caught playing with fields which the
         * log thread also manipulates.  Make sure that the buffer
         * completes any outstanding IO before proceeding. */
        rc = do_get_write_access(handle, jh, 0);
        journal_put_journal_head(jh);
        return rc;
}


/*
 * When the user wants to journal a newly created buffer_head
 * (ie. getblk() returned a new buffer and we are going to populate it
 * manually rather than reading off disk), then we need to keep the
 * buffer_head locked until it has been completely filled with new
 * data.  In this case, we should be able to make the assertion that
 * the bh is not already part of an existing transaction.
 *
 * The buffer should already be locked by the caller by this point.
 * There is no lock ranking violation: it was a newly created,
 * unlocked buffer beforehand. */

/**
 * int journal_get_create_access () - notify intent to use newly created bh
 * @handle: transaction to new buffer to
 * @bh: new buffer.
 *
 * Call this if you create a new bh.
 */
int journal_get_create_access(handle_t *handle, struct buffer_head *bh)
{
        transaction_t *transaction = handle->h_transaction;
        journal_t *journal = transaction->t_journal;
        struct journal_head *jh = journal_add_journal_head(bh);
        int err;

        jbd_debug(5, "journal_head %p\n", jh);
        err = -EROFS;
        if (is_handle_aborted(handle))
                goto out;
        err = 0;

        JBUFFER_TRACE(jh, "entry");
        /*
         * The buffer may already belong to this transaction due to pre-zeroing
         * in the filesystem's new_block code.  It may also be on the previous,
         * committing transaction's lists, but it HAS to be in Forget state in
         * that case: the transaction must have deleted the buffer for it to be
         * reused here.
         */
        jbd_lock_bh_state(bh);
        spin_lock(&journal->j_list_lock);
        J_ASSERT_JH(jh, (jh->b_transaction == transaction ||
                jh->b_transaction == NULL ||
                (jh->b_transaction == journal->j_committing_transaction &&
                          jh->b_jlist == BJ_Forget)));

        J_ASSERT_JH(jh, jh->b_next_transaction == NULL);
        J_ASSERT_JH(jh, buffer_locked(jh2bh(jh)));

        if (jh->b_transaction == NULL) {
                /*
                 * Previous journal_forget() could have left the buffer
                 * with jbddirty bit set because it was being committed. When
                 * the commit finished, we've filed the buffer for
                 * checkpointing and marked it dirty. Now we are reallocating
                 * the buffer so the transaction freeing it must have
                 * committed and so it's safe to clear the dirty bit.
                 */
                clear_buffer_dirty(jh2bh(jh));

                /* first access by this transaction */
                jh->b_modified = 0;

                JBUFFER_TRACE(jh, "file as BJ_Reserved");
                __journal_file_buffer(jh, transaction, BJ_Reserved);
        } else if (jh->b_transaction == journal->j_committing_transaction) {
                /* first access by this transaction */
                jh->b_modified = 0;

                JBUFFER_TRACE(jh, "set next transaction");
                jh->b_next_transaction = transaction;
        }
        spin_unlock(&journal->j_list_lock);
        jbd_unlock_bh_state(bh);

        /*
         * akpm: I added this.  ext3_alloc_branch can pick up new indirect
         * blocks which contain freed but then revoked metadata.  We need
         * to cancel the revoke in case we end up freeing it yet again
         * and the reallocating as data - this would cause a second revoke,
         * which hits an assertion error.
         */
        JBUFFER_TRACE(jh, "cancelling revoke");
        journal_cancel_revoke(handle, jh);
out:
        journal_put_journal_head(jh);
        return err;
}

/**
 * int journal_get_undo_access() - Notify intent to modify metadata with non-rewindable consequences
 * @handle: transaction
 * @bh: buffer to undo
 *
 * Sometimes there is a need to distinguish between metadata which has
 * been committed to disk and that which has not.  The ext3fs code uses
 * this for freeing and allocating space, we have to make sure that we
 * do not reuse freed space until the deallocation has been committed,
 * since if we overwrote that space we would make the delete
 * un-rewindable in case of a crash.
 *
 * To deal with that, journal_get_undo_access requests write access to a
 * buffer for parts of non-rewindable operations such as delete
 * operations on the bitmaps.  The journaling code must keep a copy of
 * the buffer's contents prior to the undo_access call until such time
 * as we know that the buffer has definitely been committed to disk.
 *
 * We never need to know which transaction the committed data is part
 * of, buffers touched here are guaranteed to be dirtied later and so
 * will be committed to a new transaction in due course, at which point
 * we can discard the old committed data pointer.
 *
 * Returns error number or 0 on success.
 */
int journal_get_undo_access(handle_t *handle, struct buffer_head *bh)
{
        int err;
        struct journal_head *jh = journal_add_journal_head(bh);
        char *committed_data = NULL;

        JBUFFER_TRACE(jh, "entry");

        /*
         * Do this first --- it can drop the journal lock, so we want to
         * make sure that obtaining the committed_data is done
         * atomically wrt. completion of any outstanding commits.
         */
        err = do_get_write_access(handle, jh, 1);
        if (err)
                goto out;

repeat:
        if (!jh->b_committed_data) {
                committed_data = jbd_alloc(jh2bh(jh)->b_size, GFP_NOFS);
                if (!committed_data) {
                        printk(KERN_ERR "%s: No memory for committed data\n",
                                __func__);
                        err = -ENOMEM;
                        goto out;
                }
        }

        jbd_lock_bh_state(bh);
        if (!jh->b_committed_data) {
                /* Copy out the current buffer contents into the
                 * preserved, committed copy. */
                JBUFFER_TRACE(jh, "generate b_committed data");
                if (!committed_data) {
                        jbd_unlock_bh_state(bh);
                        goto repeat;
                }

                jh->b_committed_data = committed_data;
                committed_data = NULL;
                memcpy(jh->b_committed_data, bh->b_data, bh->b_size);
        }
        jbd_unlock_bh_state(bh);
out:
        journal_put_journal_head(jh);
        if (unlikely(committed_data))
                jbd_free(committed_data, bh->b_size);
        return err;
}

/**
 * int journal_dirty_data() - mark a buffer as containing dirty data to be flushed
 * @handle: transaction
 * @bh: bufferhead to mark
 *
 * Description:
 * Mark a buffer as containing dirty data which needs to be flushed before
 * we can commit the current transaction.
 *
 * The buffer is placed on the transaction's data list and is marked as
 * belonging to the transaction.
 *
 * Returns error number or 0 on success.
 *
 * journal_dirty_data() can be called via page_launder->ext3_writepage
 * by kswapd.
 */
int journal_dirty_data(handle_t *handle, struct buffer_head *bh)
{
        journal_t *journal = handle->h_transaction->t_journal;
        int need_brelse = 0;
        struct journal_head *jh;
        int ret = 0;

        if (is_handle_aborted(handle))
                return ret;

        jh = journal_add_journal_head(bh);
        JBUFFER_TRACE(jh, "entry");

        /*
         * The buffer could *already* be dirty.  Writeout can start
         * at any time.
         */
        jbd_debug(4, "jh: %p, tid:%d\n", jh, handle->h_transaction->t_tid);

        /*
         * What if the buffer is already part of a running transaction?
         *
         * There are two cases:
         * 1) It is part of the current running transaction.  Refile it,
         *    just in case we have allocated it as metadata, deallocated
         *    it, then reallocated it as data.
         * 2) It is part of the previous, still-committing transaction.
         *    If all we want to do is to guarantee that the buffer will be
         *    written to disk before this new transaction commits, then
         *    being sure that the *previous* transaction has this same
         *    property is sufficient for us!  Just leave it on its old
         *    transaction.
         *
         * In case (2), the buffer must not already exist as metadata
         * --- that would violate write ordering (a transaction is free
         * to write its data at any point, even before the previous
         * committing transaction has committed).  The caller must
         * never, ever allow this to happen: there's nothing we can do
         * about it in this layer.
         */
        jbd_lock_bh_state(bh);
        spin_lock(&journal->j_list_lock);

        /* Now that we have bh_state locked, are we really still mapped? */
        if (!buffer_mapped(bh)) {
                JBUFFER_TRACE(jh, "unmapped buffer, bailing out");
                goto no_journal;
        }

        if (jh->b_transaction) {
                JBUFFER_TRACE(jh, "has transaction");
                if (jh->b_transaction != handle->h_transaction) {
                        JBUFFER_TRACE(jh, "belongs to older transaction");
                        J_ASSERT_JH(jh, jh->b_transaction ==
                                        journal->j_committing_transaction);

                        /* @@@ IS THIS TRUE  ? */
                        /*
                         * Not any more.  Scenario: someone does a write()
                         * in data=journal mode.  The buffer's transaction has
                         * moved into commit.  Then someone does another
                         * write() to the file.  We do the frozen data copyout
                         * and set b_next_transaction to point to j_running_t.
                         * And while we're in that state, someone does a
                         * writepage() in an attempt to pageout the same area
                         * of the file via a shared mapping.  At present that
                         * calls journal_dirty_data(), and we get right here.
                         * It may be too late to journal the data.  Simply
                         * falling through to the next test will suffice: the
                         * data will be dirty and wil be checkpointed.  The
                         * ordering comments in the next comment block still
                         * apply.
                         */
                        //J_ASSERT_JH(jh, jh->b_next_transaction == NULL);

                        /*
                         * If we're journalling data, and this buffer was
                         * subject to a write(), it could be metadata, forget
                         * or shadow against the committing transaction.  Now,
                         * someone has dirtied the same darn page via a mapping
                         * and it is being writepage()'d.
                         * We *could* just steal the page from commit, with some
                         * fancy locking there.  Instead, we just skip it -
                         * don't tie the page's buffers to the new transaction
                         * at all.
                         * Implication: if we crash before the writepage() data
                         * is written into the filesystem, recovery will replay
                         * the write() data.
                         */
                        if (jh->b_jlist != BJ_None &&
                                        jh->b_jlist != BJ_SyncData &&
                                        jh->b_jlist != BJ_Locked) {
                                JBUFFER_TRACE(jh, "Not stealing");
                                goto no_journal;
                        }

                        /*
                         * This buffer may be undergoing writeout in commit.  We
                         * can't return from here and let the caller dirty it
                         * again because that can cause the write-out loop in
                         * commit to never terminate.
                         */
                        if (buffer_dirty(bh)) {
                                get_bh(bh);
                                spin_unlock(&journal->j_list_lock);
                                jbd_unlock_bh_state(bh);
                                need_brelse = 1;
                                sync_dirty_buffer(bh);
                                jbd_lock_bh_state(bh);
                                spin_lock(&journal->j_list_lock);
                                /* Since we dropped the lock... */
                                if (!buffer_mapped(bh)) {
                                        JBUFFER_TRACE(jh, "buffer got unmapped");
                                        goto no_journal;
                                }
                                /* The buffer may become locked again at any
                                   time if it is redirtied */
                        }

                        /*
                         * We cannot remove the buffer with io error from the
                         * committing transaction, because otherwise it would
                         * miss the error and the commit would not abort.
                         */
                        if (unlikely(!buffer_uptodate(bh))) {
                                ret = -EIO;
                                goto no_journal;
                        }
                        /* We might have slept so buffer could be refiled now */
                        if (jh->b_transaction != NULL &&
                            jh->b_transaction != handle->h_transaction) {
                                JBUFFER_TRACE(jh, "unfile from commit");
                                __journal_temp_unlink_buffer(jh);
                                /* It still points to the committing
                                 * transaction; move it to this one so
                                 * that the refile assert checks are
                                 * happy. */
                                jh->b_transaction = handle->h_transaction;
                        }
                        /* The buffer will be refiled below */

                }
                /*
                 * Special case --- the buffer might actually have been
                 * allocated and then immediately deallocated in the previous,
                 * committing transaction, so might still be left on that
                 * transaction's metadata lists.
                 */
                if (jh->b_jlist != BJ_SyncData && jh->b_jlist != BJ_Locked) {
                        JBUFFER_TRACE(jh, "not on correct data list: unfile");
                        J_ASSERT_JH(jh, jh->b_jlist != BJ_Shadow);
                        JBUFFER_TRACE(jh, "file as data");
                        __journal_file_buffer(jh, handle->h_transaction,
                                                BJ_SyncData);
                }
        } else {
                JBUFFER_TRACE(jh, "not on a transaction");
                __journal_file_buffer(jh, handle->h_transaction, BJ_SyncData);
        }
no_journal:
        spin_unlock(&journal->j_list_lock);
        jbd_unlock_bh_state(bh);
        if (need_brelse) {
                BUFFER_TRACE(bh, "brelse");
                __brelse(bh);
        }
        JBUFFER_TRACE(jh, "exit");
        journal_put_journal_head(jh);
        return ret;
}

/**
 * int journal_dirty_metadata() - mark a buffer as containing dirty metadata
 * @handle: transaction to add buffer to.
 * @bh: buffer to mark
 *
 * Mark dirty metadata which needs to be journaled as part of the current
 * transaction.
 *
 * The buffer is placed on the transaction's metadata list and is marked
 * as belonging to the transaction.
 *
 * Returns error number or 0 on success.
 *
 * Special care needs to be taken if the buffer already belongs to the
 * current committing transaction (in which case we should have frozen
 * data present for that commit).  In that case, we don't relink the
 * buffer: that only gets done when the old transaction finally
 * completes its commit.
 */
int journal_dirty_metadata(handle_t *handle, struct buffer_head *bh)
{
        transaction_t *transaction = handle->h_transaction;
        journal_t *journal = transaction->t_journal;
        struct journal_head *jh = bh2jh(bh);

        jbd_debug(5, "journal_head %p\n", jh);
        JBUFFER_TRACE(jh, "entry");
        if (is_handle_aborted(handle))
                goto out;

        jbd_lock_bh_state(bh);

        if (jh->b_modified == 0) {
                /*
                 * This buffer's got modified and becoming part
                 * of the transaction. This needs to be done
                 * once a transaction -bzzz
                 */
                jh->b_modified = 1;
                J_ASSERT_JH(jh, handle->h_buffer_credits > 0);
                handle->h_buffer_credits--;
        }

        /*
         * fastpath, to avoid expensive locking.  If this buffer is already
         * on the running transaction's metadata list there is nothing to do.
         * Nobody can take it off again because there is a handle open.
         * I _think_ we're OK here with SMP barriers - a mistaken decision will
         * result in this test being false, so we go in and take the locks.
         */
        if (jh->b_transaction == transaction && jh->b_jlist == BJ_Metadata) {
                JBUFFER_TRACE(jh, "fastpath");
                J_ASSERT_JH(jh, jh->b_transaction ==
                                        journal->j_running_transaction);
                goto out_unlock_bh;
        }

        set_buffer_jbddirty(bh);

        /*
         * Metadata already on the current transaction list doesn't
         * need to be filed.  Metadata on another transaction's list must
         * be committing, and will be refiled once the commit completes:
         * leave it alone for now.
         */
        if (jh->b_transaction != transaction) {
                JBUFFER_TRACE(jh, "already on other transaction");
                J_ASSERT_JH(jh, jh->b_transaction ==
                                        journal->j_committing_transaction);
                J_ASSERT_JH(jh, jh->b_next_transaction == transaction);
                /* And this case is illegal: we can't reuse another
                 * transaction's data buffer, ever. */
                goto out_unlock_bh;
        }

        /* That test should have eliminated the following case: */
        J_ASSERT_JH(jh, jh->b_frozen_data == NULL);

        JBUFFER_TRACE(jh, "file as BJ_Metadata");
        spin_lock(&journal->j_list_lock);
        __journal_file_buffer(jh, handle->h_transaction, BJ_Metadata);
        spin_unlock(&journal->j_list_lock);
out_unlock_bh:
        jbd_unlock_bh_state(bh);
out:
        JBUFFER_TRACE(jh, "exit");
        return 0;
}

/*
 * journal_release_buffer: undo a get_write_access without any buffer
 * updates, if the update decided in the end that it didn't need access.
 *
 */
void
journal_release_buffer(handle_t *handle, struct buffer_head *bh)
{
        BUFFER_TRACE(bh, "entry");
}

/**
 * void journal_forget() - bforget() for potentially-journaled buffers.
 * @handle: transaction handle
 * @bh:     bh to 'forget'
 *
 * We can only do the bforget if there are no commits pending against the
 * buffer.  If the buffer is dirty in the current running transaction we
 * can safely unlink it.
 *
 * bh may not be a journalled buffer at all - it may be a non-JBD
 * buffer which came off the hashtable.  Check for this.
 *
 * Decrements bh->b_count by one.
 *
 * Allow this call even if the handle has aborted --- it may be part of
 * the caller's cleanup after an abort.
 */
int journal_forget (handle_t *handle, struct buffer_head *bh)
{
        transaction_t *transaction = handle->h_transaction;
        journal_t *journal = transaction->t_journal;
        struct journal_head *jh;
        int drop_reserve = 0;
        int err = 0;
        int was_modified = 0;

        BUFFER_TRACE(bh, "entry");

        jbd_lock_bh_state(bh);
        spin_lock(&journal->j_list_lock);

        if (!buffer_jbd(bh))
                goto not_jbd;
        jh = bh2jh(bh);

        /* Critical error: attempting to delete a bitmap buffer, maybe?
         * Don't do any jbd operations, and return an error. */
        if (!J_EXPECT_JH(jh, !jh->b_committed_data,
                         "inconsistent data on disk")) {
                err = -EIO;
                goto not_jbd;
        }

        /* keep track of whether or not this transaction modified us */
        was_modified = jh->b_modified;

        /*
         * The buffer's going from the transaction, we must drop
         * all references -bzzz
         */
        jh->b_modified = 0;

        if (jh->b_transaction == handle->h_transaction) {
                J_ASSERT_JH(jh, !jh->b_frozen_data);

                /* If we are forgetting a buffer which is already part
                 * of this transaction, then we can just drop it from
                 * the transaction immediately. */
                clear_buffer_dirty(bh);
                clear_buffer_jbddirty(bh);

                JBUFFER_TRACE(jh, "belongs to current transaction: unfile");

                /*
                 * we only want to drop a reference if this transaction
                 * modified the buffer
                 */
                if (was_modified)
                        drop_reserve = 1;

                /*
                 * We are no longer going to journal this buffer.
                 * However, the commit of this transaction is still
                 * important to the buffer: the delete that we are now
                 * processing might obsolete an old log entry, so by
                 * committing, we can satisfy the buffer's checkpoint.
                 *
                 * So, if we have a checkpoint on the buffer, we should
                 * now refile the buffer on our BJ_Forget list so that
                 * we know to remove the checkpoint after we commit.
                 */

                if (jh->b_cp_transaction) {
                        __journal_temp_unlink_buffer(jh);
                        __journal_file_buffer(jh, transaction, BJ_Forget);
                } else {
                        __journal_unfile_buffer(jh);
                        if (!buffer_jbd(bh)) {
                                spin_unlock(&journal->j_list_lock);
                                jbd_unlock_bh_state(bh);
                                __bforget(bh);
                                goto drop;
                        }
                }
        } else if (jh->b_transaction) {
                J_ASSERT_JH(jh, (jh->b_transaction ==
                                 journal->j_committing_transaction));
                /* However, if the buffer is still owned by a prior
                 * (committing) transaction, we can't drop it yet... */
                JBUFFER_TRACE(jh, "belongs to older transaction");
                /* ... but we CAN drop it from the new transaction if we
                 * have also modified it since the original commit. */

                if (jh->b_next_transaction) {
                        J_ASSERT(jh->b_next_transaction == transaction);
                        jh->b_next_transaction = NULL;

                        /*
                         * only drop a reference if this transaction modified
                         * the buffer
                         */
                        if (was_modified)
                                drop_reserve = 1;
                }
        }

not_jbd:
        spin_unlock(&journal->j_list_lock);
        jbd_unlock_bh_state(bh);
        __brelse(bh);
drop:
        if (drop_reserve) {
                /* no need to reserve log space for this block -bzzz */
                handle->h_buffer_credits++;
        }
        return err;
}

/**
 * int journal_stop() - complete a transaction
 * @handle: tranaction to complete.
 *
 * All done for a particular handle.
 *
 * There is not much action needed here.  We just return any remaining
 * buffer credits to the transaction and remove the handle.  The only
 * complication is that we need to start a commit operation if the
 * filesystem is marked for synchronous update.
 *
 * journal_stop itself will not usually return an error, but it may
 * do so in unusual circumstances.  In particular, expect it to
 * return -EIO if a journal_abort has been executed since the
 * transaction began.
 */
int journal_stop(handle_t *handle)
{
        transaction_t *transaction = handle->h_transaction;
        journal_t *journal = transaction->t_journal;
        int err;
        pid_t pid;

        J_ASSERT(journal_current_handle() == handle);

        if (is_handle_aborted(handle))
                err = -EIO;
        else {
                J_ASSERT(transaction->t_updates > 0);
                err = 0;
        }

        if (--handle->h_ref > 0) {
                jbd_debug(4, "h_ref %d -> %d\n", handle->h_ref + 1,
                          handle->h_ref);
                return err;
        }

        jbd_debug(4, "Handle %p going down\n", handle);

        /*
         * Implement synchronous transaction batching.  If the handle
         * was synchronous, don't force a commit immediately.  Let's
         * yield and let another thread piggyback onto this transaction.
         * Keep doing that while new threads continue to arrive.
         * It doesn't cost much - we're about to run a commit and sleep
         * on IO anyway.  Speeds up many-threaded, many-dir operations
         * by 30x or more...
         *
         * We try and optimize the sleep time against what the underlying disk
         * can do, instead of having a static sleep time.  This is useful for
         * the case where our storage is so fast that it is more optimal to go
         * ahead and force a flush and wait for the transaction to be committed
         * than it is to wait for an arbitrary amount of time for new writers to
         * join the transaction.  We achieve this by measuring how long it takes
         * to commit a transaction, and compare it with how long this
         * transaction has been running, and if run time < commit time then we
         * sleep for the delta and commit.  This greatly helps super fast disks
         * that would see slowdowns as more threads started doing fsyncs.
         *
         * But don't do this if this process was the most recent one to
         * perform a synchronous write.  We do this to detect the case where a
         * single process is doing a stream of sync writes.  No point in waiting
         * for joiners in that case.
         */
        pid = current->pid;
        if (handle->h_sync && journal->j_last_sync_writer != pid) {
                u64 commit_time, trans_time;

                journal->j_last_sync_writer = pid;

                spin_lock(&journal->j_state_lock);
                commit_time = journal->j_average_commit_time;
                spin_unlock(&journal->j_state_lock);

                trans_time = ktime_to_ns(ktime_sub(ktime_get(),
                                                   transaction->t_start_time));

                commit_time = min_t(u64, commit_time,
                                    1000*jiffies_to_usecs(1));

                if (trans_time < commit_time) {
                        ktime_t expires = ktime_add_ns(ktime_get(),
                                                       commit_time);
                        set_current_state(TASK_UNINTERRUPTIBLE);
                        schedule_hrtimeout(&expires, HRTIMER_MODE_ABS);
                }
        }

        current->journal_info = NULL;
        spin_lock(&journal->j_state_lock);
        spin_lock(&transaction->t_handle_lock);
        transaction->t_outstanding_credits -= handle->h_buffer_credits;
        transaction->t_updates--;
        if (!transaction->t_updates) {
                wake_up(&journal->j_wait_updates);
                if (journal->j_barrier_count)
                        wake_up(&journal->j_wait_transaction_locked);
        }

        /*
         * If the handle is marked SYNC, we need to set another commit
         * going!  We also want to force a commit if the current
         * transaction is occupying too much of the log, or if the
         * transaction is too old now.
         */
        if (handle->h_sync ||
                        transaction->t_outstanding_credits >
                                journal->j_max_transaction_buffers ||
                        time_after_eq(jiffies, transaction->t_expires)) {
                /* Do this even for aborted journals: an abort still
                 * completes the commit thread, it just doesn't write
                 * anything to disk. */
                tid_t tid = transaction->t_tid;

                spin_unlock(&transaction->t_handle_lock);
                jbd_debug(2, "transaction too old, requesting commit for "
                                        "handle %p\n", handle);
                /* This is non-blocking */
                __log_start_commit(journal, transaction->t_tid);
                spin_unlock(&journal->j_state_lock);

                /*
                 * Special case: JFS_SYNC synchronous updates require us
                 * to wait for the commit to complete.
                 */
                if (handle->h_sync && !(current->flags & PF_MEMALLOC))
                        err = log_wait_commit(journal, tid);
        } else {
                spin_unlock(&transaction->t_handle_lock);
                spin_unlock(&journal->j_state_lock);
        }

        lock_map_release(&handle->h_lockdep_map);

        jbd_free_handle(handle);
        return err;
}

/**
 * int journal_force_commit() - force any uncommitted transactions
 * @journal: journal to force
 *
 * For synchronous operations: force any uncommitted transactions
 * to disk.  May seem kludgy, but it reuses all the handle batching
 * code in a very simple manner.
 */
int journal_force_commit(journal_t *journal)
{
        handle_t *handle;
        int ret;

        handle = journal_start(journal, 1);
        if (IS_ERR(handle)) {
                ret = PTR_ERR(handle);
        } else {
                handle->h_sync = 1;
                ret = journal_stop(handle);
        }
        return ret;
}

/*
 *
 * List management code snippets: various functions for manipulating the
 * transaction buffer lists.
 *
 */

/*
 * Append a buffer to a transaction list, given the transaction's list head
 * pointer.
 *
 * j_list_lock is held.
 *
 * jbd_lock_bh_state(jh2bh(jh)) is held.
 */

static inline void
__blist_add_buffer(struct journal_head **list, struct journal_head *jh)
{
        if (!*list) {
                jh->b_tnext = jh->b_tprev = jh;
                *list = jh;
        } else {
                /* Insert at the tail of the list to preserve order */
                struct journal_head *first = *list, *last = first->b_tprev;
                jh->b_tprev = last;
                jh->b_tnext = first;
                last->b_tnext = first->b_tprev = jh;
        }
}

/*
 * Remove a buffer from a transaction list, given the transaction's list
 * head pointer.
 *
 * Called with j_list_lock held, and the journal may not be locked.
 *
 * jbd_lock_bh_state(jh2bh(jh)) is held.
 */

static inline void
__blist_del_buffer(struct journal_head **list, struct journal_head *jh)
{
        if (*list == jh) {
                *list = jh->b_tnext;
                if (*list == jh)
                        *list = NULL;
        }
        jh->b_tprev->b_tnext = jh->b_tnext;
        jh->b_tnext->b_tprev = jh->b_tprev;
}

/*
 * Remove a buffer from the appropriate transaction list.
 *
 * Note that this function can *change* the value of
 * bh->b_transaction->t_sync_datalist, t_buffers, t_forget,
 * t_iobuf_list, t_shadow_list, t_log_list or t_reserved_list.  If the caller
 * is holding onto a copy of one of thee pointers, it could go bad.
 * Generally the caller needs to re-read the pointer from the transaction_t.
 *
 * Called under j_list_lock.  The journal may not be locked.
 */
static void __journal_temp_unlink_buffer(struct journal_head *jh)
{
        struct journal_head **list = NULL;
        transaction_t *transaction;
        struct buffer_head *bh = jh2bh(jh);

        J_ASSERT_JH(jh, jbd_is_locked_bh_state(bh));
        transaction = jh->b_transaction;
        if (transaction)
                assert_spin_locked(&transaction->t_journal->j_list_lock);

        J_ASSERT_JH(jh, jh->b_jlist < BJ_Types);
        if (jh->b_jlist != BJ_None)
                J_ASSERT_JH(jh, transaction != NULL);

        switch (jh->b_jlist) {
        case BJ_None:
                return;
        case BJ_SyncData:
                list = &transaction->t_sync_datalist;
                break;
        case BJ_Metadata:
                transaction->t_nr_buffers--;
                J_ASSERT_JH(jh, transaction->t_nr_buffers >= 0);
                list = &transaction->t_buffers;
                break;
        case BJ_Forget:
                list = &transaction->t_forget;
                break;
        case BJ_IO:
                list = &transaction->t_iobuf_list;
                break;
        case BJ_Shadow:
                list = &transaction->t_shadow_list;
                break;
        case BJ_LogCtl:
                list = &transaction->t_log_list;
                break;
        case BJ_Reserved:
                list = &transaction->t_reserved_list;
                break;
        case BJ_Locked:
                list = &transaction->t_locked_list;
                break;
        }

        __blist_del_buffer(list, jh);
        jh->b_jlist = BJ_None;
        if (test_clear_buffer_jbddirty(bh))
                mark_buffer_dirty(bh);  /* Expose it to the VM */
}

/*
 * Remove buffer from all transactions.
 *
 * Called with bh_state lock and j_list_lock
 *
 * jh and bh may be already freed when this function returns.
 */
void __journal_unfile_buffer(struct journal_head *jh)
{
        __journal_temp_unlink_buffer(jh);
        jh->b_transaction = NULL;
        journal_put_journal_head(jh);
}

void journal_unfile_buffer(journal_t *journal, struct journal_head *jh)
{
        struct buffer_head *bh = jh2bh(jh);

        /* Get reference so that buffer cannot be freed before we unlock it */
        get_bh(bh);
        jbd_lock_bh_state(bh);
        spin_lock(&journal->j_list_lock);
        __journal_unfile_buffer(jh);
        spin_unlock(&journal->j_list_lock);
        jbd_unlock_bh_state(bh);
        __brelse(bh);
}

/*
 * Called from journal_try_to_free_buffers().
 *
 * Called under jbd_lock_bh_state(bh)
 */
static void
__journal_try_to_free_buffer(journal_t *journal, struct buffer_head *bh)
{
        struct journal_head *jh;

        jh = bh2jh(bh);

        if (buffer_locked(bh) || buffer_dirty(bh))
                goto out;

        if (jh->b_next_transaction != NULL)
                goto out;

        spin_lock(&journal->j_list_lock);
        if (jh->b_transaction != NULL && jh->b_cp_transaction == NULL) {
                if (jh->b_jlist == BJ_SyncData || jh->b_jlist == BJ_Locked) {
                        /* A written-back ordered data buffer */
                        JBUFFER_TRACE(jh, "release data");
                        __journal_unfile_buffer(jh);
                }
        } else if (jh->b_cp_transaction != NULL && jh->b_transaction == NULL) {
                /* written-back checkpointed metadata buffer */
                if (jh->b_jlist == BJ_None) {
                        JBUFFER_TRACE(jh, "remove from checkpoint list");
                        __journal_remove_checkpoint(jh);
                }
        }
        spin_unlock(&journal->j_list_lock);
out:
        return;
}

/**
 * int journal_try_to_free_buffers() - try to free page buffers.
 * @journal: journal for operation
 * @page: to try and free
 * @gfp_mask: we use the mask to detect how hard should we try to release
 * buffers. If __GFP_WAIT and __GFP_FS is set, we wait for commit code to
 * release the buffers.
 *
 *
 * For all the buffers on this page,
 * if they are fully written out ordered data, move them onto BUF_CLEAN
 * so try_to_free_buffers() can reap them.
 *
 * This function returns non-zero if we wish try_to_free_buffers()
 * to be called. We do this if the page is releasable by try_to_free_buffers().
 * We also do it if the page has locked or dirty buffers and the caller wants
 * us to perform sync or async writeout.
 *
 * This complicates JBD locking somewhat.  We aren't protected by the
 * BKL here.  We wish to remove the buffer from its committing or
 * running transaction's ->t_datalist via __journal_unfile_buffer.
 *
 * This may *change* the value of transaction_t->t_datalist, so anyone
 * who looks at t_datalist needs to lock against this function.
 *
 * Even worse, someone may be doing a journal_dirty_data on this
 * buffer.  So we need to lock against that.  journal_dirty_data()
 * will come out of the lock with the buffer dirty, which makes it
 * ineligible for release here.
 *
 * Who else is affected by this?  hmm...  Really the only contender
 * is do_get_write_access() - it could be looking at the buffer while
 * journal_try_to_free_buffer() is changing its state.  But that
 * cannot happen because we never reallocate freed data as metadata
 * while the data is part of a transaction.  Yes?
 *
 * Return 0 on failure, 1 on success
 */
int journal_try_to_free_buffers(journal_t *journal,
                                struct page *page, gfp_t gfp_mask)
{
        struct buffer_head *head;
        struct buffer_head *bh;
        int ret = 0;

        J_ASSERT(PageLocked(page));

        head = page_buffers(page);
        bh = head;
        do {
                struct journal_head *jh;

                /*
                 * We take our own ref against the journal_head here to avoid
                 * having to add tons of locking around each instance of
                 * journal_put_journal_head().
                 */
                jh = journal_grab_journal_head(bh);
                if (!jh)
                        continue;

                jbd_lock_bh_state(bh);
                __journal_try_to_free_buffer(journal, bh);
                journal_put_journal_head(jh);
                jbd_unlock_bh_state(bh);
                if (buffer_jbd(bh))
                        goto busy;
        } while ((bh = bh->b_this_page) != head);

        ret = try_to_free_buffers(page);

busy:
        return ret;
}

/*
 * This buffer is no longer needed.  If it is on an older transaction's
 * checkpoint list we need to record it on this transaction's forget list
 * to pin this buffer (and hence its checkpointing transaction) down until
 * this transaction commits.  If the buffer isn't on a checkpoint list, we
 * release it.
 * Returns non-zero if JBD no longer has an interest in the buffer.
 *
 * Called under j_list_lock.
 *
 * Called under jbd_lock_bh_state(bh).
 */
static int __dispose_buffer(struct journal_head *jh, transaction_t *transaction)
{
        int may_free = 1;
        struct buffer_head *bh = jh2bh(jh);

        if (jh->b_cp_transaction) {
                JBUFFER_TRACE(jh, "on running+cp transaction");
                __journal_temp_unlink_buffer(jh);
                /*
                 * We don't want to write the buffer anymore, clear the
                 * bit so that we don't confuse checks in
                 * __journal_file_buffer
                 */
                clear_buffer_dirty(bh);
                __journal_file_buffer(jh, transaction, BJ_Forget);
                may_free = 0;
        } else {
                JBUFFER_TRACE(jh, "on running transaction");
                __journal_unfile_buffer(jh);
        }
        return may_free;
}

/*
 * journal_invalidatepage
 *
 * This code is tricky.  It has a number of cases to deal with.
 *
 * There are two invariants which this code relies on:
 *
 * i_size must be updated on disk before we start calling invalidatepage on the
 * data.
 *
 *  This is done in ext3 by defining an ext3_setattr method which
 *  updates i_size before truncate gets going.  By maintaining this
 *  invariant, we can be sure that it is safe to throw away any buffers
 *  attached to the current transaction: once the transaction commits,
 *  we know that the data will not be needed.
 *
 *  Note however that we can *not* throw away data belonging to the
 *  previous, committing transaction!
 *
 * Any disk blocks which *are* part of the previous, committing
 * transaction (and which therefore cannot be discarded immediately) are
 * not going to be reused in the new running transaction
 *
 *  The bitmap committed_data images guarantee this: any block which is
 *  allocated in one transaction and removed in the next will be marked
 *  as in-use in the committed_data bitmap, so cannot be reused until
 *  the next transaction to delete the block commits.  This means that
 *  leaving committing buffers dirty is quite safe: the disk blocks
 *  cannot be reallocated to a different file and so buffer aliasing is
 *  not possible.
 *
 *
 * The above applies mainly to ordered data mode.  In writeback mode we
 * don't make guarantees about the order in which data hits disk --- in
 * particular we don't guarantee that new dirty data is flushed before
 * transaction commit --- so it is always safe just to discard data
 * immediately in that mode.  --sct
 */

/*
 * The journal_unmap_buffer helper function returns zero if the buffer
 * concerned remains pinned as an anonymous buffer belonging to an older
 * transaction.
 *
 * We're outside-transaction here.  Either or both of j_running_transaction
 * and j_committing_transaction may be NULL.
 */
static int journal_unmap_buffer(journal_t *journal, struct buffer_head *bh,
                                int partial_page)
{
        transaction_t *transaction;
        struct journal_head *jh;
        int may_free = 1;

        BUFFER_TRACE(bh, "entry");

retry:
        /*
         * It is safe to proceed here without the j_list_lock because the
         * buffers cannot be stolen by try_to_free_buffers as long as we are
         * holding the page lock. --sct
         */

        if (!buffer_jbd(bh))
                goto zap_buffer_unlocked;

        spin_lock(&journal->j_state_lock);
        jbd_lock_bh_state(bh);
        spin_lock(&journal->j_list_lock);

        jh = journal_grab_journal_head(bh);
        if (!jh)
                goto zap_buffer_no_jh;

        /*
         * We cannot remove the buffer from checkpoint lists until the
         * transaction adding inode to orphan list (let's call it T)
         * is committed.  Otherwise if the transaction changing the
         * buffer would be cleaned from the journal before T is
         * committed, a crash will cause that the correct contents of
         * the buffer will be lost.  On the other hand we have to
         * clear the buffer dirty bit at latest at the moment when the
         * transaction marking the buffer as freed in the filesystem
         * structures is committed because from that moment on the
         * block can be reallocated and used by a different page.
         * Since the block hasn't been freed yet but the inode has
         * already been added to orphan list, it is safe for us to add
         * the buffer to BJ_Forget list of the newest transaction.
         *
         * Also we have to clear buffer_mapped flag of a truncated buffer
         * because the buffer_head may be attached to the page straddling
         * i_size (can happen only when blocksize < pagesize) and thus the
         * buffer_head can be reused when the file is extended again. So we end
         * up keeping around invalidated buffers attached to transactions'
         * BJ_Forget list just to stop checkpointing code from cleaning up
         * the transaction this buffer was modified in.
         */
        transaction = jh->b_transaction;
        if (transaction == NULL) {
                /* First case: not on any transaction.  If it
                 * has no checkpoint link, then we can zap it:
                 * it's a writeback-mode buffer so we don't care
                 * if it hits disk safely. */
                if (!jh->b_cp_transaction) {
                        JBUFFER_TRACE(jh, "not on any transaction: zap");
                        goto zap_buffer;
                }

                if (!buffer_dirty(bh)) {
                        /* bdflush has written it.  We can drop it now */
                        goto zap_buffer;
                }

                /* OK, it must be in the journal but still not
                 * written fully to disk: it's metadata or
                 * journaled data... */

                if (journal->j_running_transaction) {
                        /* ... and once the current transaction has
                         * committed, the buffer won't be needed any
                         * longer. */
                        JBUFFER_TRACE(jh, "checkpointed: add to BJ_Forget");
                        may_free = __dispose_buffer(jh,
                                        journal->j_running_transaction);
                        goto zap_buffer;
                } else {
                        /* There is no currently-running transaction. So the
                         * orphan record which we wrote for this file must have
                         * passed into commit.  We must attach this buffer to
                         * the committing transaction, if it exists. */
                        if (journal->j_committing_transaction) {
                                JBUFFER_TRACE(jh, "give to committing trans");
                                may_free = __dispose_buffer(jh,
                                        journal->j_committing_transaction);
                                goto zap_buffer;
                        } else {
                                /* The orphan record's transaction has
                                 * committed.  We can cleanse this buffer */
                                clear_buffer_jbddirty(bh);
                                goto zap_buffer;
                        }
                }
        } else if (transaction == journal->j_committing_transaction) {
                JBUFFER_TRACE(jh, "on committing transaction");
                if (jh->b_jlist == BJ_Locked) {
                        /*
                         * The buffer is on the committing transaction's locked
                         * list.  We have the buffer locked, so I/O has
                         * completed.  So we can nail the buffer now.
                         */
                        may_free = __dispose_buffer(jh, transaction);
                        goto zap_buffer;
                }
                /*
                 * The buffer is committing, we simply cannot touch
                 * it. If the page is straddling i_size we have to wait
                 * for commit and try again.
                 */
                if (partial_page) {
                        tid_t tid = journal->j_committing_transaction->t_tid;

                        journal_put_journal_head(jh);
                        spin_unlock(&journal->j_list_lock);
                        jbd_unlock_bh_state(bh);
                        spin_unlock(&journal->j_state_lock);
                        unlock_buffer(bh);
                        log_wait_commit(journal, tid);
                        lock_buffer(bh);
                        goto retry;
                }
                /*
                 * OK, buffer won't be reachable after truncate. We just set
                 * j_next_transaction to the running transaction (if there is
                 * one) and mark buffer as freed so that commit code knows it
                 * should clear dirty bits when it is done with the buffer.
                 */
                set_buffer_freed(bh);
                if (journal->j_running_transaction && buffer_jbddirty(bh))
                        jh->b_next_transaction = journal->j_running_transaction;
                journal_put_journal_head(jh);
                spin_unlock(&journal->j_list_lock);
                jbd_unlock_bh_state(bh);
                spin_unlock(&journal->j_state_lock);
                return 0;
        } else {
                /* Good, the buffer belongs to the running transaction.
                 * We are writing our own transaction's data, not any
                 * previous one's, so it is safe to throw it away
                 * (remember that we expect the filesystem to have set
                 * i_size already for this truncate so recovery will not
                 * expose the disk blocks we are discarding here.) */
                J_ASSERT_JH(jh, transaction == journal->j_running_transaction);
                JBUFFER_TRACE(jh, "on running transaction");
                may_free = __dispose_buffer(jh, transaction);
        }

zap_buffer:
        /*
         * This is tricky. Although the buffer is truncated, it may be reused
         * if blocksize < pagesize and it is attached to the page straddling
         * EOF. Since the buffer might have been added to BJ_Forget list of the
         * running transaction, journal_get_write_access() won't clear
         * b_modified and credit accounting gets confused. So clear b_modified
         * here. */
        jh->b_modified = 0;
        journal_put_journal_head(jh);
zap_buffer_no_jh:
        spin_unlock(&journal->j_list_lock);
        jbd_unlock_bh_state(bh);
        spin_unlock(&journal->j_state_lock);
zap_buffer_unlocked:
        clear_buffer_dirty(bh);
        J_ASSERT_BH(bh, !buffer_jbddirty(bh));
        clear_buffer_mapped(bh);
        clear_buffer_req(bh);
        clear_buffer_new(bh);
        bh->b_bdev = NULL;
        return may_free;
}

/**
 * void journal_invalidatepage() - invalidate a journal page
 * @journal: journal to use for flush
 * @page:    page to flush
 * @offset:  offset of the range to invalidate
 * @length:  length of the range to invalidate
 *
 * Reap page buffers containing data in specified range in page.
 */
void journal_invalidatepage(journal_t *journal,
                      struct page *page,
                      unsigned int offset,
                      unsigned int length)
{
        struct buffer_head *head, *bh, *next;
        unsigned int stop = offset + length;
        unsigned int curr_off = 0;
        int partial_page = (offset || length < PAGE_CACHE_SIZE);
        int may_free = 1;

        if (!PageLocked(page))
                BUG();
        if (!page_has_buffers(page))
                return;

        BUG_ON(stop > PAGE_CACHE_SIZE || stop < length);

        /* We will potentially be playing with lists other than just the
         * data lists (especially for journaled data mode), so be
         * cautious in our locking. */

        head = bh = page_buffers(page);
        do {
                unsigned int next_off = curr_off + bh->b_size;
                next = bh->b_this_page;

                if (next_off > stop)
                        return;

                if (offset <= curr_off) {
                        /* This block is wholly outside the truncation point */
                        lock_buffer(bh);
                        may_free &= journal_unmap_buffer(journal, bh,
                                                         partial_page);
                        unlock_buffer(bh);
                }
                curr_off = next_off;
                bh = next;

        } while (bh != head);

        if (!partial_page) {
                if (may_free && try_to_free_buffers(page))
                        J_ASSERT(!page_has_buffers(page));
        }
}

/*
 * File a buffer on the given transaction list.
 */
void __journal_file_buffer(struct journal_head *jh,
                        transaction_t *transaction, int jlist)
{
        struct journal_head **list = NULL;
        int was_dirty = 0;
        struct buffer_head *bh = jh2bh(jh);

        J_ASSERT_JH(jh, jbd_is_locked_bh_state(bh));
        assert_spin_locked(&transaction->t_journal->j_list_lock);

        J_ASSERT_JH(jh, jh->b_jlist < BJ_Types);
        J_ASSERT_JH(jh, jh->b_transaction == transaction ||
                                jh->b_transaction == NULL);

        if (jh->b_transaction && jh->b_jlist == jlist)
                return;

        if (jlist == BJ_Metadata || jlist == BJ_Reserved ||
            jlist == BJ_Shadow || jlist == BJ_Forget) {
                /*
                 * For metadata buffers, we track dirty bit in buffer_jbddirty
                 * instead of buffer_dirty. We should not see a dirty bit set
                 * here because we clear it in do_get_write_access but e.g.
                 * tune2fs can modify the sb and set the dirty bit at any time
                 * so we try to gracefully handle that.
                 */
                if (buffer_dirty(bh))
                        warn_dirty_buffer(bh);
                if (test_clear_buffer_dirty(bh) ||
                    test_clear_buffer_jbddirty(bh))
                        was_dirty = 1;
        }

        if (jh->b_transaction)
                __journal_temp_unlink_buffer(jh);
        else
                journal_grab_journal_head(bh);
        jh->b_transaction = transaction;

        switch (jlist) {
        case BJ_None:
                J_ASSERT_JH(jh, !jh->b_committed_data);
                J_ASSERT_JH(jh, !jh->b_frozen_data);
                return;
        case BJ_SyncData:
                list = &transaction->t_sync_datalist;
                break;
        case BJ_Metadata:
                transaction->t_nr_buffers++;
                list = &transaction->t_buffers;
                break;
        case BJ_Forget:
                list = &transaction->t_forget;
                break;
        case BJ_IO:
                list = &transaction->t_iobuf_list;
                break;
        case BJ_Shadow:
                list = &transaction->t_shadow_list;
                break;
        case BJ_LogCtl:
                list = &transaction->t_log_list;
                break;
        case BJ_Reserved:
                list = &transaction->t_reserved_list;
                break;
        case BJ_Locked:
                list =  &transaction->t_locked_list;
                break;
        }

        __blist_add_buffer(list, jh);
        jh->b_jlist = jlist;

        if (was_dirty)
                set_buffer_jbddirty(bh);
}

void journal_file_buffer(struct journal_head *jh,
                                transaction_t *transaction, int jlist)
{
        jbd_lock_bh_state(jh2bh(jh));
        spin_lock(&transaction->t_journal->j_list_lock);
        __journal_file_buffer(jh, transaction, jlist);
        spin_unlock(&transaction->t_journal->j_list_lock);
        jbd_unlock_bh_state(jh2bh(jh));
}

/*
 * Remove a buffer from its current buffer list in preparation for
 * dropping it from its current transaction entirely.  If the buffer has
 * already started to be used by a subsequent transaction, refile the
 * buffer on that transaction's metadata list.
 *
 * Called under j_list_lock
 * Called under jbd_lock_bh_state(jh2bh(jh))
 *
 * jh and bh may be already free when this function returns
 */
void __journal_refile_buffer(struct journal_head *jh)
{
        int was_dirty, jlist;
        struct buffer_head *bh = jh2bh(jh);

        J_ASSERT_JH(jh, jbd_is_locked_bh_state(bh));
        if (jh->b_transaction)
                assert_spin_locked(&jh->b_transaction->t_journal->j_list_lock);

        /* If the buffer is now unused, just drop it. */
        if (jh->b_next_transaction == NULL) {
                __journal_unfile_buffer(jh);
                return;
        }

        /*
         * It has been modified by a later transaction: add it to the new
         * transaction's metadata list.
         */

        was_dirty = test_clear_buffer_jbddirty(bh);
        __journal_temp_unlink_buffer(jh);
        /*
         * We set b_transaction here because b_next_transaction will inherit
         * our jh reference and thus __journal_file_buffer() must not take a
         * new one.
         */
        jh->b_transaction = jh->b_next_transaction;
        jh->b_next_transaction = NULL;
        if (buffer_freed(bh))
                jlist = BJ_Forget;
        else if (jh->b_modified)
                jlist = BJ_Metadata;
        else
                jlist = BJ_Reserved;
        __journal_file_buffer(jh, jh->b_transaction, jlist);
        J_ASSERT_JH(jh, jh->b_transaction->t_state == T_RUNNING);

        if (was_dirty)
                set_buffer_jbddirty(bh);
}

/*
 * __journal_refile_buffer() with necessary locking added. We take our bh
 * reference so that we can safely unlock bh.
 *
 * The jh and bh may be freed by this call.
 */
void journal_refile_buffer(journal_t *journal, struct journal_head *jh)
{
        struct buffer_head *bh = jh2bh(jh);

        /* Get reference so that buffer cannot be freed before we unlock it */
        get_bh(bh);
        jbd_lock_bh_state(bh);
        spin_lock(&journal->j_list_lock);
        __journal_refile_buffer(jh);
        jbd_unlock_bh_state(bh);
        spin_unlock(&journal->j_list_lock);
        __brelse(bh);
}