btrfs: document some invariants of seed code

[mirror_ubuntu-hirsute-kernel.git] / fs / jbd2 / transaction.c

// SPDX-License-Identifier: GPL-2.0+
/*
 * linux/fs/jbd2/transaction.c
 *
 * Written by Stephen C. Tweedie <sct@redhat.com>, 1998
 *
 * Copyright 1998 Red Hat corp --- All Rights Reserved
 *
 * 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/jbd2.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>
#include <linux/backing-dev.h>
#include <linux/bug.h>
#include <linux/module.h>
#include <linux/sched/mm.h>

#include <trace/events/jbd2.h>

static void __jbd2_journal_temp_unlink_buffer(struct journal_head *jh);
static void __jbd2_journal_unfile_buffer(struct journal_head *jh);

static struct kmem_cache *transaction_cache;
int __init jbd2_journal_init_transaction_cache(void)
{
        J_ASSERT(!transaction_cache);
        transaction_cache = kmem_cache_create("jbd2_transaction_s",
                                        sizeof(transaction_t),
                                        0,
                                        SLAB_HWCACHE_ALIGN|SLAB_TEMPORARY,
                                        NULL);
        if (!transaction_cache) {
                pr_emerg("JBD2: failed to create transaction cache\n");
                return -ENOMEM;
        }
        return 0;
}

void jbd2_journal_destroy_transaction_cache(void)
{
        kmem_cache_destroy(transaction_cache);
        transaction_cache = NULL;
}

void jbd2_journal_free_transaction(transaction_t *transaction)
{
        if (unlikely(ZERO_OR_NULL_PTR(transaction)))
                return;
        kmem_cache_free(transaction_cache, transaction);
}

/*
 * Base amount of descriptor blocks we reserve for each transaction.
 */
static int jbd2_descriptor_blocks_per_trans(journal_t *journal)
{
        int tag_space = journal->j_blocksize - sizeof(journal_header_t);
        int tags_per_block;

        /* Subtract UUID */
        tag_space -= 16;
        if (jbd2_journal_has_csum_v2or3(journal))
                tag_space -= sizeof(struct jbd2_journal_block_tail);
        /* Commit code leaves a slack space of 16 bytes at the end of block */
        tags_per_block = (tag_space - 16) / journal_tag_bytes(journal);
        /*
         * Revoke descriptors are accounted separately so we need to reserve
         * space for commit block and normal transaction descriptor blocks.
         */
        return 1 + DIV_ROUND_UP(journal->j_max_transaction_buffers,
                                tags_per_block);
}

/*
 * jbd2_get_transaction: obtain a new transaction_t object.
 *
 * Simply initialise a new transaction. Initialize 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.
 *
 */

static void jbd2_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);
        atomic_set(&transaction->t_updates, 0);
        atomic_set(&transaction->t_outstanding_credits,
                   jbd2_descriptor_blocks_per_trans(journal) +
                   atomic_read(&journal->j_reserved_credits));
        atomic_set(&transaction->t_outstanding_revokes, 0);
        atomic_set(&transaction->t_handle_count, 0);
        INIT_LIST_HEAD(&transaction->t_inode_list);
        INIT_LIST_HEAD(&transaction->t_private_list);

        /* 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;
        transaction->t_max_wait = 0;
        transaction->t_start = jiffies;
        transaction->t_requested = 0;
}

/*
 * 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.
 */

/*
 * Update transaction's maximum wait time, if debugging is enabled.
 *
 * In order for t_max_wait to be reliable, it must be protected by a
 * lock.  But doing so will mean that start_this_handle() can not be
 * run in parallel on SMP systems, which limits our scalability.  So
 * unless debugging is enabled, we no longer update t_max_wait, which
 * means that maximum wait time reported by the jbd2_run_stats
 * tracepoint will always be zero.
 */
static inline void update_t_max_wait(transaction_t *transaction,
                                     unsigned long ts)
{
#ifdef CONFIG_JBD2_DEBUG
        if (jbd2_journal_enable_debug &&
            time_after(transaction->t_start, ts)) {
                ts = jbd2_time_diff(ts, transaction->t_start);
                spin_lock(&transaction->t_handle_lock);
                if (ts > transaction->t_max_wait)
                        transaction->t_max_wait = ts;
                spin_unlock(&transaction->t_handle_lock);
        }
#endif
}

/*
 * Wait until running transaction passes to T_FLUSH state and new transaction
 * can thus be started. Also starts the commit if needed. The function expects
 * running transaction to exist and releases j_state_lock.
 */
static void wait_transaction_locked(journal_t *journal)
        __releases(journal->j_state_lock)
{
        DEFINE_WAIT(wait);
        int need_to_start;
        tid_t tid = journal->j_running_transaction->t_tid;

        prepare_to_wait(&journal->j_wait_transaction_locked, &wait,
                        TASK_UNINTERRUPTIBLE);
        need_to_start = !tid_geq(journal->j_commit_request, tid);
        read_unlock(&journal->j_state_lock);
        if (need_to_start)
                jbd2_log_start_commit(journal, tid);
        jbd2_might_wait_for_commit(journal);
        schedule();
        finish_wait(&journal->j_wait_transaction_locked, &wait);
}

/*
 * Wait until running transaction transitions from T_SWITCH to T_FLUSH
 * state and new transaction can thus be started. The function releases
 * j_state_lock.
 */
static void wait_transaction_switching(journal_t *journal)
        __releases(journal->j_state_lock)
{
        DEFINE_WAIT(wait);

        if (WARN_ON(!journal->j_running_transaction ||
                    journal->j_running_transaction->t_state != T_SWITCH))
                return;
        prepare_to_wait(&journal->j_wait_transaction_locked, &wait,
                        TASK_UNINTERRUPTIBLE);
        read_unlock(&journal->j_state_lock);
        /*
         * We don't call jbd2_might_wait_for_commit() here as there's no
         * waiting for outstanding handles happening anymore in T_SWITCH state
         * and handling of reserved handles actually relies on that for
         * correctness.
         */
        schedule();
        finish_wait(&journal->j_wait_transaction_locked, &wait);
}

static void sub_reserved_credits(journal_t *journal, int blocks)
{
        atomic_sub(blocks, &journal->j_reserved_credits);
        wake_up(&journal->j_wait_reserved);
}

/*
 * Wait until we can add credits for handle to the running transaction.  Called
 * with j_state_lock held for reading. Returns 0 if handle joined the running
 * transaction. Returns 1 if we had to wait, j_state_lock is dropped, and
 * caller must retry.
 */
static int add_transaction_credits(journal_t *journal, int blocks,
                                   int rsv_blocks)
{
        transaction_t *t = journal->j_running_transaction;
        int needed;
        int total = blocks + rsv_blocks;

        /*
         * If the current transaction is locked down for commit, wait
         * for the lock to be released.
         */
        if (t->t_state != T_RUNNING) {
                WARN_ON_ONCE(t->t_state >= T_FLUSH);
                wait_transaction_locked(journal);
                return 1;
        }

        /*
         * 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.
         */
        needed = atomic_add_return(total, &t->t_outstanding_credits);
        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.
                 */
                atomic_sub(total, &t->t_outstanding_credits);

                /*
                 * Is the number of reserved credits in the current transaction too
                 * big to fit this handle? Wait until reserved credits are freed.
                 */
                if (atomic_read(&journal->j_reserved_credits) + total >
                    journal->j_max_transaction_buffers) {
                        read_unlock(&journal->j_state_lock);
                        jbd2_might_wait_for_commit(journal);
                        wait_event(journal->j_wait_reserved,
                                   atomic_read(&journal->j_reserved_credits) + total <=
                                   journal->j_max_transaction_buffers);
                        return 1;
                }

                wait_transaction_locked(journal);
                return 1;
        }

        /*
         * 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.
         */
        if (jbd2_log_space_left(journal) < journal->j_max_transaction_buffers) {
                atomic_sub(total, &t->t_outstanding_credits);
                read_unlock(&journal->j_state_lock);
                jbd2_might_wait_for_commit(journal);
                write_lock(&journal->j_state_lock);
                if (jbd2_log_space_left(journal) <
                                        journal->j_max_transaction_buffers)
                        __jbd2_log_wait_for_space(journal);
                write_unlock(&journal->j_state_lock);
                return 1;
        }

        /* No reservation? We are done... */
        if (!rsv_blocks)
                return 0;

        needed = atomic_add_return(rsv_blocks, &journal->j_reserved_credits);
        /* We allow at most half of a transaction to be reserved */
        if (needed > journal->j_max_transaction_buffers / 2) {
                sub_reserved_credits(journal, rsv_blocks);
                atomic_sub(total, &t->t_outstanding_credits);
                read_unlock(&journal->j_state_lock);
                jbd2_might_wait_for_commit(journal);
                wait_event(journal->j_wait_reserved,
                         atomic_read(&journal->j_reserved_credits) + rsv_blocks
                         <= journal->j_max_transaction_buffers / 2);
                return 1;
        }
        return 0;
}

/*
 * 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,
                             gfp_t gfp_mask)
{
        transaction_t   *transaction, *new_transaction = NULL;
        int             blocks = handle->h_total_credits;
        int             rsv_blocks = 0;
        unsigned long ts = jiffies;

        if (handle->h_rsv_handle)
                rsv_blocks = handle->h_rsv_handle->h_total_credits;

        /*
         * Limit the number of reserved credits to 1/2 of maximum transaction
         * size and limit the number of total credits to not exceed maximum
         * transaction size per operation.
         */
        if ((rsv_blocks > journal->j_max_transaction_buffers / 2) ||
            (rsv_blocks + blocks > journal->j_max_transaction_buffers)) {
                printk(KERN_ERR "JBD2: %s wants too many credits "
                       "credits:%d rsv_credits:%d max:%d\n",
                       current->comm, blocks, rsv_blocks,
                       journal->j_max_transaction_buffers);
                WARN_ON(1);
                return -ENOSPC;
        }

alloc_transaction:
        if (!journal->j_running_transaction) {
                /*
                 * If __GFP_FS is not present, then we may be being called from
                 * inside the fs writeback layer, so we MUST NOT fail.
                 */
                if ((gfp_mask & __GFP_FS) == 0)
                        gfp_mask |= __GFP_NOFAIL;
                new_transaction = kmem_cache_zalloc(transaction_cache,
                                                    gfp_mask);
                if (!new_transaction)
                        return -ENOMEM;
        }

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

        /*
         * We need to hold j_state_lock until t_updates has been incremented,
         * for proper journal barrier handling
         */
repeat:
        read_lock(&journal->j_state_lock);
        BUG_ON(journal->j_flags & JBD2_UNMOUNT);
        if (is_journal_aborted(journal) ||
            (journal->j_errno != 0 && !(journal->j_flags & JBD2_ACK_ERR))) {
                read_unlock(&journal->j_state_lock);
                jbd2_journal_free_transaction(new_transaction);
                return -EROFS;
        }

        /*
         * Wait on the journal's transaction barrier if necessary. Specifically
         * we allow reserved handles to proceed because otherwise commit could
         * deadlock on page writeback not being able to complete.
         */
        if (!handle->h_reserved && journal->j_barrier_count) {
                read_unlock(&journal->j_state_lock);
                wait_event(journal->j_wait_transaction_locked,
                                journal->j_barrier_count == 0);
                goto repeat;
        }

        if (!journal->j_running_transaction) {
                read_unlock(&journal->j_state_lock);
                if (!new_transaction)
                        goto alloc_transaction;
                write_lock(&journal->j_state_lock);
                if (!journal->j_running_transaction &&
                    (handle->h_reserved || !journal->j_barrier_count)) {
                        jbd2_get_transaction(journal, new_transaction);
                        new_transaction = NULL;
                }
                write_unlock(&journal->j_state_lock);
                goto repeat;
        }

        transaction = journal->j_running_transaction;

        if (!handle->h_reserved) {
                /* We may have dropped j_state_lock - restart in that case */
                if (add_transaction_credits(journal, blocks, rsv_blocks))
                        goto repeat;
        } else {
                /*
                 * We have handle reserved so we are allowed to join T_LOCKED
                 * transaction and we don't have to check for transaction size
                 * and journal space. But we still have to wait while running
                 * transaction is being switched to a committing one as it
                 * won't wait for any handles anymore.
                 */
                if (transaction->t_state == T_SWITCH) {
                        wait_transaction_switching(journal);
                        goto repeat;
                }
                sub_reserved_credits(journal, blocks);
                handle->h_reserved = 0;
        }

        /* OK, account for the buffers that this operation expects to
         * use and add the handle to the running transaction. 
         */
        update_t_max_wait(transaction, ts);
        handle->h_transaction = transaction;
        handle->h_requested_credits = blocks;
        handle->h_revoke_credits_requested = handle->h_revoke_credits;
        handle->h_start_jiffies = jiffies;
        atomic_inc(&transaction->t_updates);
        atomic_inc(&transaction->t_handle_count);
        jbd_debug(4, "Handle %p given %d credits (total %d, free %lu)\n",
                  handle, blocks,
                  atomic_read(&transaction->t_outstanding_credits),
                  jbd2_log_space_left(journal));
        read_unlock(&journal->j_state_lock);
        current->journal_info = handle;

        rwsem_acquire_read(&journal->j_trans_commit_map, 0, 0, _THIS_IP_);
        jbd2_journal_free_transaction(new_transaction);
        /*
         * Ensure that no allocations done while the transaction is open are
         * going to recurse back to the fs layer.
         */
        handle->saved_alloc_context = memalloc_nofs_save();
        return 0;
}

/* Allocate a new handle.  This should probably be in a slab... */
static handle_t *new_handle(int nblocks)
{
        handle_t *handle = jbd2_alloc_handle(GFP_NOFS);
        if (!handle)
                return NULL;
        handle->h_total_credits = nblocks;
        handle->h_ref = 1;

        return handle;
}

handle_t *jbd2__journal_start(journal_t *journal, int nblocks, int rsv_blocks,
                              int revoke_records, gfp_t gfp_mask,
                              unsigned int type, unsigned int line_no)
{
        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;
        }

        nblocks += DIV_ROUND_UP(revoke_records,
                                journal->j_revoke_records_per_block);
        handle = new_handle(nblocks);
        if (!handle)
                return ERR_PTR(-ENOMEM);
        if (rsv_blocks) {
                handle_t *rsv_handle;

                rsv_handle = new_handle(rsv_blocks);
                if (!rsv_handle) {
                        jbd2_free_handle(handle);
                        return ERR_PTR(-ENOMEM);
                }
                rsv_handle->h_reserved = 1;
                rsv_handle->h_journal = journal;
                handle->h_rsv_handle = rsv_handle;
        }
        handle->h_revoke_credits = revoke_records;

        err = start_this_handle(journal, handle, gfp_mask);
        if (err < 0) {
                if (handle->h_rsv_handle)
                        jbd2_free_handle(handle->h_rsv_handle);
                jbd2_free_handle(handle);
                return ERR_PTR(err);
        }
        handle->h_type = type;
        handle->h_line_no = line_no;
        trace_jbd2_handle_start(journal->j_fs_dev->bd_dev,
                                handle->h_transaction->t_tid, type,
                                line_no, nblocks);

        return handle;
}
EXPORT_SYMBOL(jbd2__journal_start);


/**
 * handle_t *jbd2_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. Additionally, if rsv_blocks > 0, we also create another
 * handle with rsv_blocks reserved blocks in the journal. This handle is
 * stored in h_rsv_handle. It is not attached to any particular transaction
 * and thus doesn't block transaction commit. If the caller uses this reserved
 * handle, it has to set h_rsv_handle to NULL as otherwise jbd2_journal_stop()
 * on the parent handle will dispose the reserved one. Reserved handle has to
 * be converted to a normal handle using jbd2_journal_start_reserved() before
 * it can be used.
 *
 * Return a pointer to a newly allocated handle, or an ERR_PTR() value
 * on failure.
 */
handle_t *jbd2_journal_start(journal_t *journal, int nblocks)
{
        return jbd2__journal_start(journal, nblocks, 0, 0, GFP_NOFS, 0, 0);
}
EXPORT_SYMBOL(jbd2_journal_start);

static void __jbd2_journal_unreserve_handle(handle_t *handle, transaction_t *t)
{
        journal_t *journal = handle->h_journal;

        WARN_ON(!handle->h_reserved);
        sub_reserved_credits(journal, handle->h_total_credits);
        if (t)
                atomic_sub(handle->h_total_credits, &t->t_outstanding_credits);
}

void jbd2_journal_free_reserved(handle_t *handle)
{
        journal_t *journal = handle->h_journal;

        /* Get j_state_lock to pin running transaction if it exists */
        read_lock(&journal->j_state_lock);
        __jbd2_journal_unreserve_handle(handle, journal->j_running_transaction);
        read_unlock(&journal->j_state_lock);
        jbd2_free_handle(handle);
}
EXPORT_SYMBOL(jbd2_journal_free_reserved);

/**
 * int jbd2_journal_start_reserved() - start reserved handle
 * @handle: handle to start
 * @type: for handle statistics
 * @line_no: for handle statistics
 *
 * Start handle that has been previously reserved with jbd2_journal_reserve().
 * This attaches @handle to the running transaction (or creates one if there's
 * not transaction running). Unlike jbd2_journal_start() this function cannot
 * block on journal commit, checkpointing, or similar stuff. It can block on
 * memory allocation or frozen journal though.
 *
 * Return 0 on success, non-zero on error - handle is freed in that case.
 */
int jbd2_journal_start_reserved(handle_t *handle, unsigned int type,
                                unsigned int line_no)
{
        journal_t *journal = handle->h_journal;
        int ret = -EIO;

        if (WARN_ON(!handle->h_reserved)) {
                /* Someone passed in normal handle? Just stop it. */
                jbd2_journal_stop(handle);
                return ret;
        }
        /*
         * Usefulness of mixing of reserved and unreserved handles is
         * questionable. So far nobody seems to need it so just error out.
         */
        if (WARN_ON(current->journal_info)) {
                jbd2_journal_free_reserved(handle);
                return ret;
        }

        handle->h_journal = NULL;
        /*
         * GFP_NOFS is here because callers are likely from writeback or
         * similarly constrained call sites
         */
        ret = start_this_handle(journal, handle, GFP_NOFS);
        if (ret < 0) {
                handle->h_journal = journal;
                jbd2_journal_free_reserved(handle);
                return ret;
        }
        handle->h_type = type;
        handle->h_line_no = line_no;
        trace_jbd2_handle_start(journal->j_fs_dev->bd_dev,
                                handle->h_transaction->t_tid, type,
                                line_no, handle->h_total_credits);
        return 0;
}
EXPORT_SYMBOL(jbd2_journal_start_reserved);

/**
 * int jbd2_journal_extend() - extend buffer credits.
 * @handle:  handle to 'extend'
 * @nblocks: nr blocks to try to extend by.
 * @revoke_records: number of revoke records 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 modifications in advance, but can
 * extend its credit if it needs more.
 *
 * jbd2_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 jbd2_journal_extend(handle_t *handle, int nblocks, int revoke_records)
{
        transaction_t *transaction = handle->h_transaction;
        journal_t *journal;
        int result;
        int wanted;

        if (is_handle_aborted(handle))
                return -EROFS;
        journal = transaction->t_journal;

        result = 1;

        read_lock(&journal->j_state_lock);

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

        nblocks += DIV_ROUND_UP(
                        handle->h_revoke_credits_requested + revoke_records,
                        journal->j_revoke_records_per_block) -
                DIV_ROUND_UP(
                        handle->h_revoke_credits_requested,
                        journal->j_revoke_records_per_block);
        spin_lock(&transaction->t_handle_lock);
        wanted = atomic_add_return(nblocks,
                                   &transaction->t_outstanding_credits);

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

        trace_jbd2_handle_extend(journal->j_fs_dev->bd_dev,
                                 transaction->t_tid,
                                 handle->h_type, handle->h_line_no,
                                 handle->h_total_credits,
                                 nblocks);

        handle->h_total_credits += nblocks;
        handle->h_requested_credits += nblocks;
        handle->h_revoke_credits += revoke_records;
        handle->h_revoke_credits_requested += revoke_records;
        result = 0;

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

static void stop_this_handle(handle_t *handle)
{
        transaction_t *transaction = handle->h_transaction;
        journal_t *journal = transaction->t_journal;
        int revokes;

        J_ASSERT(journal_current_handle() == handle);
        J_ASSERT(atomic_read(&transaction->t_updates) > 0);
        current->journal_info = NULL;
        /*
         * Subtract necessary revoke descriptor blocks from handle credits. We
         * take care to account only for revoke descriptor blocks the
         * transaction will really need as large sequences of transactions with
         * small numbers of revokes are relatively common.
         */
        revokes = handle->h_revoke_credits_requested - handle->h_revoke_credits;
        if (revokes) {
                int t_revokes, revoke_descriptors;
                int rr_per_blk = journal->j_revoke_records_per_block;

                WARN_ON_ONCE(DIV_ROUND_UP(revokes, rr_per_blk)
                                > handle->h_total_credits);
                t_revokes = atomic_add_return(revokes,
                                &transaction->t_outstanding_revokes);
                revoke_descriptors =
                        DIV_ROUND_UP(t_revokes, rr_per_blk) -
                        DIV_ROUND_UP(t_revokes - revokes, rr_per_blk);
                handle->h_total_credits -= revoke_descriptors;
        }
        atomic_sub(handle->h_total_credits,
                   &transaction->t_outstanding_credits);
        if (handle->h_rsv_handle)
                __jbd2_journal_unreserve_handle(handle->h_rsv_handle,
                                                transaction);
        if (atomic_dec_and_test(&transaction->t_updates))
                wake_up(&journal->j_wait_updates);

        rwsem_release(&journal->j_trans_commit_map, _THIS_IP_);
        /*
         * Scope of the GFP_NOFS context is over here and so we can restore the
         * original alloc context.
         */
        memalloc_nofs_restore(handle->saved_alloc_context);
}

/**
 * int jbd2_journal_restart() - restart a handle .
 * @handle:  handle to restart
 * @nblocks: nr credits requested
 * @revoke_records: number of revoke record credits requested
 * @gfp_mask: memory allocation flags (for start_this_handle)
 *
 * Restart a handle for a multi-transaction filesystem
 * operation.
 *
 * If the jbd2_journal_extend() call above fails to grant new buffer credits
 * to a running handle, a call to jbd2_journal_restart will commit the
 * handle's transaction so far and reattach the handle to a new
 * transaction capable of guaranteeing the requested number of
 * credits. We preserve reserved handle if there's any attached to the
 * passed in handle.
 */
int jbd2__journal_restart(handle_t *handle, int nblocks, int revoke_records,
                          gfp_t gfp_mask)
{
        transaction_t *transaction = handle->h_transaction;
        journal_t *journal;
        tid_t           tid;
        int             need_to_start;
        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;
        journal = transaction->t_journal;
        tid = transaction->t_tid;

        /*
         * First unlink the handle from its current transaction, and start the
         * commit on that.
         */
        jbd_debug(2, "restarting handle %p\n", handle);
        stop_this_handle(handle);
        handle->h_transaction = NULL;

        /*
         * TODO: If we use READ_ONCE / WRITE_ONCE for j_commit_request we can
         * get rid of pointless j_state_lock traffic like this.
         */
        read_lock(&journal->j_state_lock);
        need_to_start = !tid_geq(journal->j_commit_request, tid);
        read_unlock(&journal->j_state_lock);
        if (need_to_start)
                jbd2_log_start_commit(journal, tid);
        handle->h_total_credits = nblocks +
                DIV_ROUND_UP(revoke_records,
                             journal->j_revoke_records_per_block);
        handle->h_revoke_credits = revoke_records;
        ret = start_this_handle(journal, handle, gfp_mask);
        trace_jbd2_handle_restart(journal->j_fs_dev->bd_dev,
                                 ret ? 0 : handle->h_transaction->t_tid,
                                 handle->h_type, handle->h_line_no,
                                 handle->h_total_credits);
        return ret;
}
EXPORT_SYMBOL(jbd2__journal_restart);


int jbd2_journal_restart(handle_t *handle, int nblocks)
{
        return jbd2__journal_restart(handle, nblocks, 0, GFP_NOFS);
}
EXPORT_SYMBOL(jbd2_journal_restart);

/**
 * void jbd2_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.
 *
 * The journal lock should not be held on entry.
 */
void jbd2_journal_lock_updates(journal_t *journal)
{
        DEFINE_WAIT(wait);

        jbd2_might_wait_for_commit(journal);

        write_lock(&journal->j_state_lock);
        ++journal->j_barrier_count;

        /* Wait until there are no reserved handles */
        if (atomic_read(&journal->j_reserved_credits)) {
                write_unlock(&journal->j_state_lock);
                wait_event(journal->j_wait_reserved,
                           atomic_read(&journal->j_reserved_credits) == 0);
                write_lock(&journal->j_state_lock);
        }

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

        /*
         * We have now established a barrier against other normal updates, but
         * we also need to barrier against other jbd2_journal_lock_updates() calls
         * to make sure that we serialise special journal-locked operations
         * too.
         */
        mutex_lock(&journal->j_barrier);
}

/**
 * void jbd2_journal_unlock_updates (journal_t* journal) - release barrier
 * @journal:  Journal to release the barrier on.
 *
 * Release a transaction barrier obtained with jbd2_journal_lock_updates().
 *
 * Should be called without the journal lock held.
 */
void jbd2_journal_unlock_updates (journal_t *journal)
{
        J_ASSERT(journal->j_barrier_count != 0);

        mutex_unlock(&journal->j_barrier);
        write_lock(&journal->j_state_lock);
        --journal->j_barrier_count;
        write_unlock(&journal->j_state_lock);
        wake_up(&journal->j_wait_transaction_locked);
}

static void warn_dirty_buffer(struct buffer_head *bh)
{
        printk(KERN_WARNING
               "JBD2: Spotted dirty metadata buffer (dev = %pg, blocknr = %llu). "
               "There's a risk of filesystem corruption in case of system "
               "crash.\n",
               bh->b_bdev, (unsigned long long)bh->b_blocknr);
}

/* Call t_frozen trigger and copy buffer data into jh->b_frozen_data. */
static void jbd2_freeze_jh_data(struct journal_head *jh)
{
        struct page *page;
        int offset;
        char *source;
        struct buffer_head *bh = jh2bh(jh);

        J_EXPECT_JH(jh, buffer_uptodate(bh), "Possible IO failure.\n");
        page = bh->b_page;
        offset = offset_in_page(bh->b_data);
        source = kmap_atomic(page);
        /* Fire data frozen trigger just before we copy the data */
        jbd2_buffer_frozen_trigger(jh, source + offset, jh->b_triggers);
        memcpy(jh->b_frozen_data, source + offset, bh->b_size);
        kunmap_atomic(source);

        /*
         * Now that the frozen data is saved off, we need to store any matching
         * triggers.
         */
        jh->b_frozen_triggers = jh->b_triggers;
}

/*
 * 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 = handle->h_transaction;
        journal_t *journal;
        int error;
        char *frozen_buffer = NULL;
        unsigned long start_lock, time_lock;

        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. */

        start_lock = jiffies;
        lock_buffer(bh);
        spin_lock(&jh->b_state_lock);

        /* If it takes too long to lock the buffer, trace it */
        time_lock = jbd2_time_diff(start_lock, jiffies);
        if (time_lock > HZ/10)
                trace_jbd2_lock_buffer_stall(bh->b_bdev->bd_dev,
                        jiffies_to_msecs(time_lock));

        /* 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)) {
                spin_unlock(&jh->b_state_lock);
                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 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");
                /*
                 * Make sure all stores to jh (b_modified, b_frozen_data) are
                 * visible before attaching it to the running transaction.
                 * Paired with barrier in jbd2_write_access_granted()
                 */
                smp_wmb();
                spin_lock(&journal->j_list_lock);
                __jbd2_journal_file_buffer(jh, transaction, BJ_Reserved);
                spin_unlock(&journal->j_list_lock);
                goto done;
        }
        /*
         * 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);
                goto attach_next;
        }

        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 (buffer_shadow(bh)) {
                JBUFFER_TRACE(jh, "on shadow: sleep");
                spin_unlock(&jh->b_state_lock);
                wait_on_bit_io(&bh->b_state, BH_Shadow, TASK_UNINTERRUPTIBLE);
                goto repeat;
        }

        /*
         * Only do the copy if the currently-owning transaction still needs it.
         * If buffer isn't on BJ_Metadata list, the committing transaction is
         * past that stage (here we use the fact that BH_Shadow is set under
         * bh_state lock together with refiling to BJ_Shadow list and at this
         * point we know the buffer doesn't have BH_Shadow set).
         *
         * 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_Metadata || force_copy) {
                JBUFFER_TRACE(jh, "generate frozen data");
                if (!frozen_buffer) {
                        JBUFFER_TRACE(jh, "allocate memory for buffer");
                        spin_unlock(&jh->b_state_lock);
                        frozen_buffer = jbd2_alloc(jh2bh(jh)->b_size,
                                                   GFP_NOFS | __GFP_NOFAIL);
                        goto repeat;
                }
                jh->b_frozen_data = frozen_buffer;
                frozen_buffer = NULL;
                jbd2_freeze_jh_data(jh);
        }
attach_next:
        /*
         * Make sure all stores to jh (b_modified, b_frozen_data) are visible
         * before attaching it to the running transaction. Paired with barrier
         * in jbd2_write_access_granted()
         */
        smp_wmb();
        jh->b_next_transaction = transaction;

done:
        spin_unlock(&jh->b_state_lock);

        /*
         * If we are about to journal a buffer, then any revoke pending on it is
         * no longer valid
         */
        jbd2_journal_cancel_revoke(handle, jh);

out:
        if (unlikely(frozen_buffer))    /* It's usually NULL */
                jbd2_free(frozen_buffer, bh->b_size);

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

/* Fast check whether buffer is already attached to the required transaction */
static bool jbd2_write_access_granted(handle_t *handle, struct buffer_head *bh,
                                                        bool undo)
{
        struct journal_head *jh;
        bool ret = false;

        /* Dirty buffers require special handling... */
        if (buffer_dirty(bh))
                return false;

        /*
         * RCU protects us from dereferencing freed pages. So the checks we do
         * are guaranteed not to oops. However the jh slab object can get freed
         * & reallocated while we work with it. So we have to be careful. When
         * we see jh attached to the running transaction, we know it must stay
         * so until the transaction is committed. Thus jh won't be freed and
         * will be attached to the same bh while we run.  However it can
         * happen jh gets freed, reallocated, and attached to the transaction
         * just after we get pointer to it from bh. So we have to be careful
         * and recheck jh still belongs to our bh before we return success.
         */
        rcu_read_lock();
        if (!buffer_jbd(bh))
                goto out;
        /* This should be bh2jh() but that doesn't work with inline functions */
        jh = READ_ONCE(bh->b_private);
        if (!jh)
                goto out;
        /* For undo access buffer must have data copied */
        if (undo && !jh->b_committed_data)
                goto out;
        if (READ_ONCE(jh->b_transaction) != handle->h_transaction &&
            READ_ONCE(jh->b_next_transaction) != handle->h_transaction)
                goto out;
        /*
         * There are two reasons for the barrier here:
         * 1) Make sure to fetch b_bh after we did previous checks so that we
         * detect when jh went through free, realloc, attach to transaction
         * while we were checking. Paired with implicit barrier in that path.
         * 2) So that access to bh done after jbd2_write_access_granted()
         * doesn't get reordered and see inconsistent state of concurrent
         * do_get_write_access().
         */
        smp_mb();
        if (unlikely(jh->b_bh != bh))
                goto out;
        ret = true;
out:
        rcu_read_unlock();
        return ret;
}

/**
 * int jbd2_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: 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 jbd2_journal_get_write_access(handle_t *handle, struct buffer_head *bh)
{
        struct journal_head *jh;
        int rc;

        if (is_handle_aborted(handle))
                return -EROFS;

        if (jbd2_write_access_granted(handle, bh, false))
                return 0;

        jh = jbd2_journal_add_journal_head(bh);
        /* 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);
        jbd2_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 jbd2_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 jbd2_journal_get_create_access(handle_t *handle, struct buffer_head *bh)
{
        transaction_t *transaction = handle->h_transaction;
        journal_t *journal;
        struct journal_head *jh = jbd2_journal_add_journal_head(bh);
        int err;

        jbd_debug(5, "journal_head %p\n", jh);
        err = -EROFS;
        if (is_handle_aborted(handle))
                goto out;
        journal = transaction->t_journal;
        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.
         */
        spin_lock(&jh->b_state_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 jbd2_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");
                spin_lock(&journal->j_list_lock);
                __jbd2_journal_file_buffer(jh, transaction, BJ_Reserved);
                spin_unlock(&journal->j_list_lock);
        } else if (jh->b_transaction == journal->j_committing_transaction) {
                /* first access by this transaction */
                jh->b_modified = 0;

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

        /*
         * 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");
        jbd2_journal_cancel_revoke(handle, jh);
out:
        jbd2_journal_put_journal_head(jh);
        return err;
}

/**
 * int jbd2_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, jbd2_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 jbd2_journal_get_undo_access(handle_t *handle, struct buffer_head *bh)
{
        int err;
        struct journal_head *jh;
        char *committed_data = NULL;

        if (is_handle_aborted(handle))
                return -EROFS;

        if (jbd2_write_access_granted(handle, bh, true))
                return 0;

        jh = jbd2_journal_add_journal_head(bh);
        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 = jbd2_alloc(jh2bh(jh)->b_size,
                                            GFP_NOFS|__GFP_NOFAIL);

        spin_lock(&jh->b_state_lock);
        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) {
                        spin_unlock(&jh->b_state_lock);
                        goto repeat;
                }

                jh->b_committed_data = committed_data;
                committed_data = NULL;
                memcpy(jh->b_committed_data, bh->b_data, bh->b_size);
        }
        spin_unlock(&jh->b_state_lock);
out:
        jbd2_journal_put_journal_head(jh);
        if (unlikely(committed_data))
                jbd2_free(committed_data, bh->b_size);
        return err;
}

/**
 * void jbd2_journal_set_triggers() - Add triggers for commit writeout
 * @bh: buffer to trigger on
 * @type: struct jbd2_buffer_trigger_type containing the trigger(s).
 *
 * Set any triggers on this journal_head.  This is always safe, because
 * triggers for a committing buffer will be saved off, and triggers for
 * a running transaction will match the buffer in that transaction.
 *
 * Call with NULL to clear the triggers.
 */
void jbd2_journal_set_triggers(struct buffer_head *bh,
                               struct jbd2_buffer_trigger_type *type)
{
        struct journal_head *jh = jbd2_journal_grab_journal_head(bh);

        if (WARN_ON(!jh))
                return;
        jh->b_triggers = type;
        jbd2_journal_put_journal_head(jh);
}

void jbd2_buffer_frozen_trigger(struct journal_head *jh, void *mapped_data,
                                struct jbd2_buffer_trigger_type *triggers)
{
        struct buffer_head *bh = jh2bh(jh);

        if (!triggers || !triggers->t_frozen)
                return;

        triggers->t_frozen(triggers, bh, mapped_data, bh->b_size);
}

void jbd2_buffer_abort_trigger(struct journal_head *jh,
                               struct jbd2_buffer_trigger_type *triggers)
{
        if (!triggers || !triggers->t_abort)
                return;

        triggers->t_abort(triggers, jh2bh(jh));
}

/**
 * int jbd2_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 must have previously had jbd2_journal_get_write_access()
 * called so that it has a valid journal_head attached to the buffer
 * head.
 *
 * 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 jbd2_journal_dirty_metadata(handle_t *handle, struct buffer_head *bh)
{
        transaction_t *transaction = handle->h_transaction;
        journal_t *journal;
        struct journal_head *jh;
        int ret = 0;

        if (is_handle_aborted(handle))
                return -EROFS;
        if (!buffer_jbd(bh))
                return -EUCLEAN;

        /*
         * We don't grab jh reference here since the buffer must be part
         * of the running transaction.
         */
        jh = bh2jh(bh);
        jbd_debug(5, "journal_head %p\n", jh);
        JBUFFER_TRACE(jh, "entry");

        /*
         * This and the following assertions are unreliable since we may see jh
         * in inconsistent state unless we grab bh_state lock. But this is
         * crucial to catch bugs so let's do a reliable check until the
         * lockless handling is fully proven.
         */
        if (jh->b_transaction != transaction &&
            jh->b_next_transaction != transaction) {
                spin_lock(&jh->b_state_lock);
                J_ASSERT_JH(jh, jh->b_transaction == transaction ||
                                jh->b_next_transaction == transaction);
                spin_unlock(&jh->b_state_lock);
        }
        if (jh->b_modified == 1) {
                /* If it's in our transaction it must be in BJ_Metadata list. */
                if (jh->b_transaction == transaction &&
                    jh->b_jlist != BJ_Metadata) {
                        spin_lock(&jh->b_state_lock);
                        if (jh->b_transaction == transaction &&
                            jh->b_jlist != BJ_Metadata)
                                pr_err("JBD2: assertion failure: h_type=%u "
                                       "h_line_no=%u block_no=%llu jlist=%u\n",
                                       handle->h_type, handle->h_line_no,
                                       (unsigned long long) bh->b_blocknr,
                                       jh->b_jlist);
                        J_ASSERT_JH(jh, jh->b_transaction != transaction ||
                                        jh->b_jlist == BJ_Metadata);
                        spin_unlock(&jh->b_state_lock);
                }
                goto out;
        }

        journal = transaction->t_journal;
        spin_lock(&jh->b_state_lock);

        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
                 */
                if (WARN_ON_ONCE(jbd2_handle_buffer_credits(handle) <= 0)) {
                        ret = -ENOSPC;
                        goto out_unlock_bh;
                }
                jh->b_modified = 1;
                handle->h_total_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");
                if (unlikely(jh->b_transaction !=
                             journal->j_running_transaction)) {
                        printk(KERN_ERR "JBD2: %s: "
                               "jh->b_transaction (%llu, %p, %u) != "
                               "journal->j_running_transaction (%p, %u)\n",
                               journal->j_devname,
                               (unsigned long long) bh->b_blocknr,
                               jh->b_transaction,
                               jh->b_transaction ? jh->b_transaction->t_tid : 0,
                               journal->j_running_transaction,
                               journal->j_running_transaction ?
                               journal->j_running_transaction->t_tid : 0);
                        ret = -EINVAL;
                }
                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");
                if (unlikely(((jh->b_transaction !=
                               journal->j_committing_transaction)) ||
                             (jh->b_next_transaction != transaction))) {
                        printk(KERN_ERR "jbd2_journal_dirty_metadata: %s: "
                               "bad jh for block %llu: "
                               "transaction (%p, %u), "
                               "jh->b_transaction (%p, %u), "
                               "jh->b_next_transaction (%p, %u), jlist %u\n",
                               journal->j_devname,
                               (unsigned long long) bh->b_blocknr,
                               transaction, transaction->t_tid,
                               jh->b_transaction,
                               jh->b_transaction ?
                               jh->b_transaction->t_tid : 0,
                               jh->b_next_transaction,
                               jh->b_next_transaction ?
                               jh->b_next_transaction->t_tid : 0,
                               jh->b_jlist);
                        WARN_ON(1);
                        ret = -EINVAL;
                }
                /* 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);
        __jbd2_journal_file_buffer(jh, transaction, BJ_Metadata);
        spin_unlock(&journal->j_list_lock);
out_unlock_bh:
        spin_unlock(&jh->b_state_lock);
out:
        JBUFFER_TRACE(jh, "exit");
        return ret;
}

/**
 * void jbd2_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 jbd2_journal_forget(handle_t *handle, struct buffer_head *bh)
{
        transaction_t *transaction = handle->h_transaction;
        journal_t *journal;
        struct journal_head *jh;
        int drop_reserve = 0;
        int err = 0;
        int was_modified = 0;

        if (is_handle_aborted(handle))
                return -EROFS;
        journal = transaction->t_journal;

        BUFFER_TRACE(bh, "entry");

        jh = jbd2_journal_grab_journal_head(bh);
        if (!jh) {
                __bforget(bh);
                return 0;
        }

        spin_lock(&jh->b_state_lock);

        /* 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 drop;
        }

        /* 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 == 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.
                 */

                spin_lock(&journal->j_list_lock);
                if (jh->b_cp_transaction) {
                        __jbd2_journal_temp_unlink_buffer(jh);
                        __jbd2_journal_file_buffer(jh, transaction, BJ_Forget);
                } else {
                        __jbd2_journal_unfile_buffer(jh);
                        jbd2_journal_put_journal_head(jh);
                }
                spin_unlock(&journal->j_list_lock);
        } 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 through
                 * marking the buffer as freed and set j_next_transaction to
                 * the new transaction, so that not only the commit code
                 * knows it should clear dirty bits when it is done with the
                 * buffer, but also the buffer can be checkpointed only
                 * after the new transaction commits. */

                set_buffer_freed(bh);

                if (!jh->b_next_transaction) {
                        spin_lock(&journal->j_list_lock);
                        jh->b_next_transaction = transaction;
                        spin_unlock(&journal->j_list_lock);
                } else {
                        J_ASSERT(jh->b_next_transaction == transaction);

                        /*
                         * only drop a reference if this transaction modified
                         * the buffer
                         */
                        if (was_modified)
                                drop_reserve = 1;
                }
        } else {
                /*
                 * Finally, if the buffer is not belongs to any
                 * transaction, we can just drop it now if it has no
                 * checkpoint.
                 */
                spin_lock(&journal->j_list_lock);
                if (!jh->b_cp_transaction) {
                        JBUFFER_TRACE(jh, "belongs to none transaction");
                        spin_unlock(&journal->j_list_lock);
                        goto drop;
                }

                /*
                 * Otherwise, if the buffer has been written to disk,
                 * it is safe to remove the checkpoint and drop it.
                 */
                if (!buffer_dirty(bh)) {
                        __jbd2_journal_remove_checkpoint(jh);
                        spin_unlock(&journal->j_list_lock);
                        goto drop;
                }

                /*
                 * The buffer is still not written to disk, we should
                 * attach this buffer to current transaction so that the
                 * buffer can be checkpointed only after the current
                 * transaction commits.
                 */
                clear_buffer_dirty(bh);
                __jbd2_journal_file_buffer(jh, transaction, BJ_Forget);
                spin_unlock(&journal->j_list_lock);
        }
drop:
        __brelse(bh);
        spin_unlock(&jh->b_state_lock);
        jbd2_journal_put_journal_head(jh);
        if (drop_reserve) {
                /* no need to reserve log space for this block -bzzz */
                handle->h_total_credits++;
        }
        return err;
}

/**
 * int jbd2_journal_stop() - complete a transaction
 * @handle: transaction 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.
 *
 * jbd2_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 jbd2_journal_abort has been executed since the
 * transaction began.
 */
int jbd2_journal_stop(handle_t *handle)
{
        transaction_t *transaction = handle->h_transaction;
        journal_t *journal;
        int err = 0, wait_for_commit = 0;
        tid_t tid;
        pid_t pid;

        if (--handle->h_ref > 0) {
                jbd_debug(4, "h_ref %d -> %d\n", handle->h_ref + 1,
                                                 handle->h_ref);
                if (is_handle_aborted(handle))
                        return -EIO;
                return 0;
        }
        if (!transaction) {
                /*
                 * Handle is already detached from the transaction so there is
                 * nothing to do other than free the handle.
                 */
                memalloc_nofs_restore(handle->saved_alloc_context);
                goto free_and_exit;
        }
        journal = transaction->t_journal;
        tid = transaction->t_tid;

        if (is_handle_aborted(handle))
                err = -EIO;

        jbd_debug(4, "Handle %p going down\n", handle);
        trace_jbd2_handle_stats(journal->j_fs_dev->bd_dev,
                                tid, handle->h_type, handle->h_line_no,
                                jiffies - handle->h_start_jiffies,
                                handle->h_sync, handle->h_requested_credits,
                                (handle->h_requested_credits -
                                 handle->h_total_credits));

        /*
         * 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.
         *
         * Setting max_batch_time to 0 disables this completely.
         */
        pid = current->pid;
        if (handle->h_sync && journal->j_last_sync_writer != pid &&
            journal->j_max_batch_time) {
                u64 commit_time, trans_time;

                journal->j_last_sync_writer = pid;

                read_lock(&journal->j_state_lock);
                commit_time = journal->j_average_commit_time;
                read_unlock(&journal->j_state_lock);

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

                commit_time = max_t(u64, commit_time,
                                    1000*journal->j_min_batch_time);
                commit_time = min_t(u64, commit_time,
                                    1000*journal->j_max_batch_time);

                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);
                }
        }

        if (handle->h_sync)
                transaction->t_synchronous_commit = 1;

        /*
         * If the handle is marked SYNC, we need to set another commit
         * going!  We also want to force a commit if the transaction is too
         * old now.
         */
        if (handle->h_sync ||
            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. */

                jbd_debug(2, "transaction too old, requesting commit for "
                                        "handle %p\n", handle);
                /* This is non-blocking */
                jbd2_log_start_commit(journal, tid);

                /*
                 * Special case: JBD2_SYNC synchronous updates require us
                 * to wait for the commit to complete.
                 */
                if (handle->h_sync && !(current->flags & PF_MEMALLOC))
                        wait_for_commit = 1;
        }

        /*
         * Once stop_this_handle() drops t_updates, the transaction could start
         * committing on us and eventually disappear.  So we must not
         * dereference transaction pointer again after calling
         * stop_this_handle().
         */
        stop_this_handle(handle);

        if (wait_for_commit)
                err = jbd2_log_wait_commit(journal, tid);

free_and_exit:
        if (handle->h_rsv_handle)
                jbd2_free_handle(handle->h_rsv_handle);
        jbd2_free_handle(handle);
        return err;
}

/*
 *
 * 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.
 *
 * jh->b_state_lock 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.
 *
 * jh->b_state_lock 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_buffers, t_forget, t_shadow_list, t_log_list or
 * t_reserved_list.  If the caller is holding onto a copy of one of these
 * pointers, it could go bad.  Generally the caller needs to re-read the
 * pointer from the transaction_t.
 *
 * Called under j_list_lock.
 */
static void __jbd2_journal_temp_unlink_buffer(struct journal_head *jh)
{
        struct journal_head **list = NULL;
        transaction_t *transaction;
        struct buffer_head *bh = jh2bh(jh);

        lockdep_assert_held(&jh->b_state_lock);
        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_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_Shadow:
                list = &transaction->t_shadow_list;
                break;
        case BJ_Reserved:
                list = &transaction->t_reserved_list;
                break;
        }

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

/*
 * Remove buffer from all transactions. The caller is responsible for dropping
 * the jh reference that belonged to the transaction.
 *
 * Called with bh_state lock and j_list_lock
 */
static void __jbd2_journal_unfile_buffer(struct journal_head *jh)
{
        J_ASSERT_JH(jh, jh->b_transaction != NULL);
        J_ASSERT_JH(jh, jh->b_next_transaction == NULL);

        __jbd2_journal_temp_unlink_buffer(jh);
        jh->b_transaction = NULL;
}

void jbd2_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);
        spin_lock(&jh->b_state_lock);
        spin_lock(&journal->j_list_lock);
        __jbd2_journal_unfile_buffer(jh);
        spin_unlock(&journal->j_list_lock);
        spin_unlock(&jh->b_state_lock);
        jbd2_journal_put_journal_head(jh);
        __brelse(bh);
}

/*
 * Called from jbd2_journal_try_to_free_buffers().
 *
 * Called under jh->b_state_lock
 */
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 || jh->b_transaction != NULL)
                goto out;

        spin_lock(&journal->j_list_lock);
        if (jh->b_cp_transaction != NULL) {
                /* written-back checkpointed metadata buffer */
                JBUFFER_TRACE(jh, "remove from checkpoint list");
                __jbd2_journal_remove_checkpoint(jh);
        }
        spin_unlock(&journal->j_list_lock);
out:
        return;
}

/**
 * int jbd2_journal_try_to_free_buffers() - try to free page buffers.
 * @journal: journal for operation
 * @page: to try and free
 *
 * 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 __jbd2_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 jbd2_journal_dirty_data on this
 * buffer.  So we need to lock against that.  jbd2_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 jbd2_journal_try_to_free_buffers(journal_t *journal, struct page *page)
{
        struct buffer_head *head;
        struct buffer_head *bh;
        bool has_write_io_error = false;
        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
                 * jbd2_journal_put_journal_head().
                 */
                jh = jbd2_journal_grab_journal_head(bh);
                if (!jh)
                        continue;

                spin_lock(&jh->b_state_lock);
                __journal_try_to_free_buffer(journal, bh);
                spin_unlock(&jh->b_state_lock);
                jbd2_journal_put_journal_head(jh);
                if (buffer_jbd(bh))
                        goto busy;

                /*
                 * If we free a metadata buffer which has been failed to
                 * write out, the jbd2 checkpoint procedure will not detect
                 * this failure and may lead to filesystem inconsistency
                 * after cleanup journal tail.
                 */
                if (buffer_write_io_error(bh)) {
                        pr_err("JBD2: Error while async write back metadata bh %llu.",
                               (unsigned long long)bh->b_blocknr);
                        has_write_io_error = true;
                }
        } while ((bh = bh->b_this_page) != head);

        ret = try_to_free_buffers(page);

busy:
        if (has_write_io_error)
                jbd2_journal_abort(journal, -EIO);

        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 jh->b_state_lock.
 */
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");
                __jbd2_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);
                __jbd2_journal_file_buffer(jh, transaction, BJ_Forget);
                may_free = 0;
        } else {
                JBUFFER_TRACE(jh, "on running transaction");
                __jbd2_journal_unfile_buffer(jh);
                jbd2_journal_put_journal_head(jh);
        }
        return may_free;
}

/*
 * jbd2_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");

        /*
         * 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
         */

        jh = jbd2_journal_grab_journal_head(bh);
        if (!jh)
                goto zap_buffer_unlocked;

        /* OK, we have data buffer in journaled mode */
        write_lock(&journal->j_state_lock);
        spin_lock(&jh->b_state_lock);
        spin_lock(&journal->j_list_lock);

        /*
         * 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 */
                        __jbd2_journal_remove_checkpoint(jh);
                        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);
                                __jbd2_journal_remove_checkpoint(jh);
                                goto zap_buffer;
                        }
                }
        } else if (transaction == journal->j_committing_transaction) {
                JBUFFER_TRACE(jh, "on committing transaction");
                /*
                 * 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) {
                        spin_unlock(&journal->j_list_lock);
                        spin_unlock(&jh->b_state_lock);
                        write_unlock(&journal->j_state_lock);
                        jbd2_journal_put_journal_head(jh);
                        return -EBUSY;
                }
                /*
                 * OK, buffer won't be reachable after truncate. We just clear
                 * b_modified to not confuse transaction credit accounting, and
                 * 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;
                jh->b_modified = 0;
                spin_unlock(&journal->j_list_lock);
                spin_unlock(&jh->b_state_lock);
                write_unlock(&journal->j_state_lock);
                jbd2_journal_put_journal_head(jh);
                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;
        spin_unlock(&journal->j_list_lock);
        spin_unlock(&jh->b_state_lock);
        write_unlock(&journal->j_state_lock);
        jbd2_journal_put_journal_head(jh);
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);
        clear_buffer_delay(bh);
        clear_buffer_unwritten(bh);
        bh->b_bdev = NULL;
        return may_free;
}

/**
 * void jbd2_journal_invalidatepage()
 * @journal: journal to use for flush...
 * @page:    page to flush
 * @offset:  start of the range to invalidate
 * @length:  length of the range to invalidate
 *
 * Reap page buffers containing data after in the specified range in page.
 * Can return -EBUSY if buffers are part of the committing transaction and
 * the page is straddling i_size. Caller then has to wait for current commit
 * and try again.
 */
int jbd2_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_SIZE);
        int may_free = 1;
        int ret = 0;

        if (!PageLocked(page))
                BUG();
        if (!page_has_buffers(page))
                return 0;

        BUG_ON(stop > PAGE_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 0;

                if (offset <= curr_off) {
                        /* This block is wholly outside the truncation point */
                        lock_buffer(bh);
                        ret = journal_unmap_buffer(journal, bh, partial_page);
                        unlock_buffer(bh);
                        if (ret < 0)
                                return ret;
                        may_free &= ret;
                }
                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));
        }
        return 0;
}

/*
 * File a buffer on the given transaction list.
 */
void __jbd2_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);

        lockdep_assert_held(&jh->b_state_lock);
        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)
                __jbd2_journal_temp_unlink_buffer(jh);
        else
                jbd2_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_Metadata:
                transaction->t_nr_buffers++;
                list = &transaction->t_buffers;
                break;
        case BJ_Forget:
                list = &transaction->t_forget;
                break;
        case BJ_Shadow:
                list = &transaction->t_shadow_list;
                break;
        case BJ_Reserved:
                list = &transaction->t_reserved_list;
                break;
        }

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

        if (was_dirty)
                set_buffer_jbddirty(bh);
}

void jbd2_journal_file_buffer(struct journal_head *jh,
                                transaction_t *transaction, int jlist)
{
        spin_lock(&jh->b_state_lock);
        spin_lock(&transaction->t_journal->j_list_lock);
        __jbd2_journal_file_buffer(jh, transaction, jlist);
        spin_unlock(&transaction->t_journal->j_list_lock);
        spin_unlock(&jh->b_state_lock);
}

/*
 * 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 jh->b_state_lock
 *
 * When this function returns true, there's no next transaction to refile to
 * and the caller has to drop jh reference through
 * jbd2_journal_put_journal_head().
 */
bool __jbd2_journal_refile_buffer(struct journal_head *jh)
{
        int was_dirty, jlist;
        struct buffer_head *bh = jh2bh(jh);

        lockdep_assert_held(&jh->b_state_lock);
        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) {
                __jbd2_journal_unfile_buffer(jh);
                return true;
        }

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

        was_dirty = test_clear_buffer_jbddirty(bh);
        __jbd2_journal_temp_unlink_buffer(jh);

        /*
         * b_transaction must be set, otherwise the new b_transaction won't
         * be holding jh reference
         */
        J_ASSERT_JH(jh, jh->b_transaction != NULL);

        /*
         * We set b_transaction here because b_next_transaction will inherit
         * our jh reference and thus __jbd2_journal_file_buffer() must not
         * take a new one.
         */
        WRITE_ONCE(jh->b_transaction, jh->b_next_transaction);
        WRITE_ONCE(jh->b_next_transaction, NULL);
        if (buffer_freed(bh))
                jlist = BJ_Forget;
        else if (jh->b_modified)
                jlist = BJ_Metadata;
        else
                jlist = BJ_Reserved;
        __jbd2_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);
        return false;
}

/*
 * __jbd2_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 jbd2_journal_refile_buffer(journal_t *journal, struct journal_head *jh)
{
        bool drop;

        spin_lock(&jh->b_state_lock);
        spin_lock(&journal->j_list_lock);
        drop = __jbd2_journal_refile_buffer(jh);
        spin_unlock(&jh->b_state_lock);
        spin_unlock(&journal->j_list_lock);
        if (drop)
                jbd2_journal_put_journal_head(jh);
}

/*
 * File inode in the inode list of the handle's transaction
 */
static int jbd2_journal_file_inode(handle_t *handle, struct jbd2_inode *jinode,
                unsigned long flags, loff_t start_byte, loff_t end_byte)
{
        transaction_t *transaction = handle->h_transaction;
        journal_t *journal;

        if (is_handle_aborted(handle))
                return -EROFS;
        journal = transaction->t_journal;

        jbd_debug(4, "Adding inode %lu, tid:%d\n", jinode->i_vfs_inode->i_ino,
                        transaction->t_tid);

        spin_lock(&journal->j_list_lock);
        jinode->i_flags |= flags;

        if (jinode->i_dirty_end) {
                jinode->i_dirty_start = min(jinode->i_dirty_start, start_byte);
                jinode->i_dirty_end = max(jinode->i_dirty_end, end_byte);
        } else {
                jinode->i_dirty_start = start_byte;
                jinode->i_dirty_end = end_byte;
        }

        /* Is inode already attached where we need it? */
        if (jinode->i_transaction == transaction ||
            jinode->i_next_transaction == transaction)
                goto done;

        /*
         * We only ever set this variable to 1 so the test is safe. Since
         * t_need_data_flush is likely to be set, we do the test to save some
         * cacheline bouncing
         */
        if (!transaction->t_need_data_flush)
                transaction->t_need_data_flush = 1;
        /* On some different transaction's list - should be
         * the committing one */
        if (jinode->i_transaction) {
                J_ASSERT(jinode->i_next_transaction == NULL);
                J_ASSERT(jinode->i_transaction ==
                                        journal->j_committing_transaction);
                jinode->i_next_transaction = transaction;
                goto done;
        }
        /* Not on any transaction list... */
        J_ASSERT(!jinode->i_next_transaction);
        jinode->i_transaction = transaction;
        list_add(&jinode->i_list, &transaction->t_inode_list);
done:
        spin_unlock(&journal->j_list_lock);

        return 0;
}

int jbd2_journal_inode_ranged_write(handle_t *handle,
                struct jbd2_inode *jinode, loff_t start_byte, loff_t length)
{
        return jbd2_journal_file_inode(handle, jinode,
                        JI_WRITE_DATA | JI_WAIT_DATA, start_byte,
                        start_byte + length - 1);
}

int jbd2_journal_inode_ranged_wait(handle_t *handle, struct jbd2_inode *jinode,
                loff_t start_byte, loff_t length)
{
        return jbd2_journal_file_inode(handle, jinode, JI_WAIT_DATA,
                        start_byte, start_byte + length - 1);
}

/*
 * File truncate and transaction commit interact with each other in a
 * non-trivial way.  If a transaction writing data block A is
 * committing, we cannot discard the data by truncate until we have
 * written them.  Otherwise if we crashed after the transaction with
 * write has committed but before the transaction with truncate has
 * committed, we could see stale data in block A.  This function is a
 * helper to solve this problem.  It starts writeout of the truncated
 * part in case it is in the committing transaction.
 *
 * Filesystem code must call this function when inode is journaled in
 * ordered mode before truncation happens and after the inode has been
 * placed on orphan list with the new inode size. The second condition
 * avoids the race that someone writes new data and we start
 * committing the transaction after this function has been called but
 * before a transaction for truncate is started (and furthermore it
 * allows us to optimize the case where the addition to orphan list
 * happens in the same transaction as write --- we don't have to write
 * any data in such case).
 */
int jbd2_journal_begin_ordered_truncate(journal_t *journal,
                                        struct jbd2_inode *jinode,
                                        loff_t new_size)
{
        transaction_t *inode_trans, *commit_trans;
        int ret = 0;

        /* This is a quick check to avoid locking if not necessary */
        if (!jinode->i_transaction)
                goto out;
        /* Locks are here just to force reading of recent values, it is
         * enough that the transaction was not committing before we started
         * a transaction adding the inode to orphan list */
        read_lock(&journal->j_state_lock);
        commit_trans = journal->j_committing_transaction;
        read_unlock(&journal->j_state_lock);
        spin_lock(&journal->j_list_lock);
        inode_trans = jinode->i_transaction;
        spin_unlock(&journal->j_list_lock);
        if (inode_trans == commit_trans) {
                ret = filemap_fdatawrite_range(jinode->i_vfs_inode->i_mapping,
                        new_size, LLONG_MAX);
                if (ret)
                        jbd2_journal_abort(journal, ret);
        }
out:
        return ret;
}