btrfs: do not start relocation until in progress drops are done

[mirror_ubuntu-jammy-kernel.git] / fs / btrfs / transaction.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Copyright (C) 2007 Oracle.  All rights reserved.
 */

#include <linux/fs.h>
#include <linux/slab.h>
#include <linux/sched.h>
#include <linux/writeback.h>
#include <linux/pagemap.h>
#include <linux/blkdev.h>
#include <linux/uuid.h>
#include "misc.h"
#include "ctree.h"
#include "disk-io.h"
#include "transaction.h"
#include "locking.h"
#include "tree-log.h"
#include "volumes.h"
#include "dev-replace.h"
#include "qgroup.h"
#include "block-group.h"
#include "space-info.h"
#include "zoned.h"

#define BTRFS_ROOT_TRANS_TAG 0

/*
 * Transaction states and transitions
 *
 * No running transaction (fs tree blocks are not modified)
 * |
 * | To next stage:
 * |  Call start_transaction() variants. Except btrfs_join_transaction_nostart().
 * V
 * Transaction N [[TRANS_STATE_RUNNING]]
 * |
 * | New trans handles can be attached to transaction N by calling all
 * | start_transaction() variants.
 * |
 * | To next stage:
 * |  Call btrfs_commit_transaction() on any trans handle attached to
 * |  transaction N
 * V
 * Transaction N [[TRANS_STATE_COMMIT_START]]
 * |
 * | Will wait for previous running transaction to completely finish if there
 * | is one
 * |
 * | Then one of the following happes:
 * | - Wait for all other trans handle holders to release.
 * |   The btrfs_commit_transaction() caller will do the commit work.
 * | - Wait for current transaction to be committed by others.
 * |   Other btrfs_commit_transaction() caller will do the commit work.
 * |
 * | At this stage, only btrfs_join_transaction*() variants can attach
 * | to this running transaction.
 * | All other variants will wait for current one to finish and attach to
 * | transaction N+1.
 * |
 * | To next stage:
 * |  Caller is chosen to commit transaction N, and all other trans handle
 * |  haven been released.
 * V
 * Transaction N [[TRANS_STATE_COMMIT_DOING]]
 * |
 * | The heavy lifting transaction work is started.
 * | From running delayed refs (modifying extent tree) to creating pending
 * | snapshots, running qgroups.
 * | In short, modify supporting trees to reflect modifications of subvolume
 * | trees.
 * |
 * | At this stage, all start_transaction() calls will wait for this
 * | transaction to finish and attach to transaction N+1.
 * |
 * | To next stage:
 * |  Until all supporting trees are updated.
 * V
 * Transaction N [[TRANS_STATE_UNBLOCKED]]
 * |                                                Transaction N+1
 * | All needed trees are modified, thus we only    [[TRANS_STATE_RUNNING]]
 * | need to write them back to disk and update     |
 * | super blocks.                                  |
 * |                                                |
 * | At this stage, new transaction is allowed to   |
 * | start.                                         |
 * | All new start_transaction() calls will be      |
 * | attached to transid N+1.                       |
 * |                                                |
 * | To next stage:                                 |
 * |  Until all tree blocks are super blocks are    |
 * |  written to block devices                      |
 * V                                                |
 * Transaction N [[TRANS_STATE_COMPLETED]]          V
 *   All tree blocks and super blocks are written.  Transaction N+1
 *   This transaction is finished and all its       [[TRANS_STATE_COMMIT_START]]
 *   data structures will be cleaned up.            | Life goes on
 */
static const unsigned int btrfs_blocked_trans_types[TRANS_STATE_MAX] = {
        [TRANS_STATE_RUNNING]           = 0U,
        [TRANS_STATE_COMMIT_START]      = (__TRANS_START | __TRANS_ATTACH),
        [TRANS_STATE_COMMIT_DOING]      = (__TRANS_START |
                                           __TRANS_ATTACH |
                                           __TRANS_JOIN |
                                           __TRANS_JOIN_NOSTART),
        [TRANS_STATE_UNBLOCKED]         = (__TRANS_START |
                                           __TRANS_ATTACH |
                                           __TRANS_JOIN |
                                           __TRANS_JOIN_NOLOCK |
                                           __TRANS_JOIN_NOSTART),
        [TRANS_STATE_SUPER_COMMITTED]   = (__TRANS_START |
                                           __TRANS_ATTACH |
                                           __TRANS_JOIN |
                                           __TRANS_JOIN_NOLOCK |
                                           __TRANS_JOIN_NOSTART),
        [TRANS_STATE_COMPLETED]         = (__TRANS_START |
                                           __TRANS_ATTACH |
                                           __TRANS_JOIN |
                                           __TRANS_JOIN_NOLOCK |
                                           __TRANS_JOIN_NOSTART),
};

void btrfs_put_transaction(struct btrfs_transaction *transaction)
{
        WARN_ON(refcount_read(&transaction->use_count) == 0);
        if (refcount_dec_and_test(&transaction->use_count)) {
                BUG_ON(!list_empty(&transaction->list));
                WARN_ON(!RB_EMPTY_ROOT(
                                &transaction->delayed_refs.href_root.rb_root));
                WARN_ON(!RB_EMPTY_ROOT(
                                &transaction->delayed_refs.dirty_extent_root));
                if (transaction->delayed_refs.pending_csums)
                        btrfs_err(transaction->fs_info,
                                  "pending csums is %llu",
                                  transaction->delayed_refs.pending_csums);
                /*
                 * If any block groups are found in ->deleted_bgs then it's
                 * because the transaction was aborted and a commit did not
                 * happen (things failed before writing the new superblock
                 * and calling btrfs_finish_extent_commit()), so we can not
                 * discard the physical locations of the block groups.
                 */
                while (!list_empty(&transaction->deleted_bgs)) {
                        struct btrfs_block_group *cache;

                        cache = list_first_entry(&transaction->deleted_bgs,
                                                 struct btrfs_block_group,
                                                 bg_list);
                        list_del_init(&cache->bg_list);
                        btrfs_unfreeze_block_group(cache);
                        btrfs_put_block_group(cache);
                }
                WARN_ON(!list_empty(&transaction->dev_update_list));
                kfree(transaction);
        }
}

static noinline void switch_commit_roots(struct btrfs_trans_handle *trans)
{
        struct btrfs_transaction *cur_trans = trans->transaction;
        struct btrfs_fs_info *fs_info = trans->fs_info;
        struct btrfs_root *root, *tmp;
        struct btrfs_caching_control *caching_ctl, *next;

        down_write(&fs_info->commit_root_sem);
        list_for_each_entry_safe(root, tmp, &cur_trans->switch_commits,
                                 dirty_list) {
                list_del_init(&root->dirty_list);
                free_extent_buffer(root->commit_root);
                root->commit_root = btrfs_root_node(root);
                extent_io_tree_release(&root->dirty_log_pages);
                btrfs_qgroup_clean_swapped_blocks(root);
        }

        /* We can free old roots now. */
        spin_lock(&cur_trans->dropped_roots_lock);
        while (!list_empty(&cur_trans->dropped_roots)) {
                root = list_first_entry(&cur_trans->dropped_roots,
                                        struct btrfs_root, root_list);
                list_del_init(&root->root_list);
                spin_unlock(&cur_trans->dropped_roots_lock);
                btrfs_free_log(trans, root);
                btrfs_drop_and_free_fs_root(fs_info, root);
                spin_lock(&cur_trans->dropped_roots_lock);
        }
        spin_unlock(&cur_trans->dropped_roots_lock);

        /*
         * We have to update the last_byte_to_unpin under the commit_root_sem,
         * at the same time we swap out the commit roots.
         *
         * This is because we must have a real view of the last spot the caching
         * kthreads were while caching.  Consider the following views of the
         * extent tree for a block group
         *
         * commit root
         * +----+----+----+----+----+----+----+
         * |\\\\|    |\\\\|\\\\|    |\\\\|\\\\|
         * +----+----+----+----+----+----+----+
         * 0    1    2    3    4    5    6    7
         *
         * new commit root
         * +----+----+----+----+----+----+----+
         * |    |    |    |\\\\|    |    |\\\\|
         * +----+----+----+----+----+----+----+
         * 0    1    2    3    4    5    6    7
         *
         * If the cache_ctl->progress was at 3, then we are only allowed to
         * unpin [0,1) and [2,3], because the caching thread has already
         * processed those extents.  We are not allowed to unpin [5,6), because
         * the caching thread will re-start it's search from 3, and thus find
         * the hole from [4,6) to add to the free space cache.
         */
        spin_lock(&fs_info->block_group_cache_lock);
        list_for_each_entry_safe(caching_ctl, next,
                                 &fs_info->caching_block_groups, list) {
                struct btrfs_block_group *cache = caching_ctl->block_group;

                if (btrfs_block_group_done(cache)) {
                        cache->last_byte_to_unpin = (u64)-1;
                        list_del_init(&caching_ctl->list);
                        btrfs_put_caching_control(caching_ctl);
                } else {
                        cache->last_byte_to_unpin = caching_ctl->progress;
                }
        }
        spin_unlock(&fs_info->block_group_cache_lock);
        up_write(&fs_info->commit_root_sem);
}

static inline void extwriter_counter_inc(struct btrfs_transaction *trans,
                                         unsigned int type)
{
        if (type & TRANS_EXTWRITERS)
                atomic_inc(&trans->num_extwriters);
}

static inline void extwriter_counter_dec(struct btrfs_transaction *trans,
                                         unsigned int type)
{
        if (type & TRANS_EXTWRITERS)
                atomic_dec(&trans->num_extwriters);
}

static inline void extwriter_counter_init(struct btrfs_transaction *trans,
                                          unsigned int type)
{
        atomic_set(&trans->num_extwriters, ((type & TRANS_EXTWRITERS) ? 1 : 0));
}

static inline int extwriter_counter_read(struct btrfs_transaction *trans)
{
        return atomic_read(&trans->num_extwriters);
}

/*
 * To be called after doing the chunk btree updates right after allocating a new
 * chunk (after btrfs_chunk_alloc_add_chunk_item() is called), when removing a
 * chunk after all chunk btree updates and after finishing the second phase of
 * chunk allocation (btrfs_create_pending_block_groups()) in case some block
 * group had its chunk item insertion delayed to the second phase.
 */
void btrfs_trans_release_chunk_metadata(struct btrfs_trans_handle *trans)
{
        struct btrfs_fs_info *fs_info = trans->fs_info;

        if (!trans->chunk_bytes_reserved)
                return;

        btrfs_block_rsv_release(fs_info, &fs_info->chunk_block_rsv,
                                trans->chunk_bytes_reserved, NULL);
        trans->chunk_bytes_reserved = 0;
}

/*
 * either allocate a new transaction or hop into the existing one
 */
static noinline int join_transaction(struct btrfs_fs_info *fs_info,
                                     unsigned int type)
{
        struct btrfs_transaction *cur_trans;

        spin_lock(&fs_info->trans_lock);
loop:
        /* The file system has been taken offline. No new transactions. */
        if (test_bit(BTRFS_FS_STATE_ERROR, &fs_info->fs_state)) {
                spin_unlock(&fs_info->trans_lock);
                return -EROFS;
        }

        cur_trans = fs_info->running_transaction;
        if (cur_trans) {
                if (TRANS_ABORTED(cur_trans)) {
                        spin_unlock(&fs_info->trans_lock);
                        return cur_trans->aborted;
                }
                if (btrfs_blocked_trans_types[cur_trans->state] & type) {
                        spin_unlock(&fs_info->trans_lock);
                        return -EBUSY;
                }
                refcount_inc(&cur_trans->use_count);
                atomic_inc(&cur_trans->num_writers);
                extwriter_counter_inc(cur_trans, type);
                spin_unlock(&fs_info->trans_lock);
                return 0;
        }
        spin_unlock(&fs_info->trans_lock);

        /*
         * If we are ATTACH, we just want to catch the current transaction,
         * and commit it. If there is no transaction, just return ENOENT.
         */
        if (type == TRANS_ATTACH)
                return -ENOENT;

        /*
         * JOIN_NOLOCK only happens during the transaction commit, so
         * it is impossible that ->running_transaction is NULL
         */
        BUG_ON(type == TRANS_JOIN_NOLOCK);

        cur_trans = kmalloc(sizeof(*cur_trans), GFP_NOFS);
        if (!cur_trans)
                return -ENOMEM;

        spin_lock(&fs_info->trans_lock);
        if (fs_info->running_transaction) {
                /*
                 * someone started a transaction after we unlocked.  Make sure
                 * to redo the checks above
                 */
                kfree(cur_trans);
                goto loop;
        } else if (test_bit(BTRFS_FS_STATE_ERROR, &fs_info->fs_state)) {
                spin_unlock(&fs_info->trans_lock);
                kfree(cur_trans);
                return -EROFS;
        }

        cur_trans->fs_info = fs_info;
        atomic_set(&cur_trans->pending_ordered, 0);
        init_waitqueue_head(&cur_trans->pending_wait);
        atomic_set(&cur_trans->num_writers, 1);
        extwriter_counter_init(cur_trans, type);
        init_waitqueue_head(&cur_trans->writer_wait);
        init_waitqueue_head(&cur_trans->commit_wait);
        cur_trans->state = TRANS_STATE_RUNNING;
        /*
         * One for this trans handle, one so it will live on until we
         * commit the transaction.
         */
        refcount_set(&cur_trans->use_count, 2);
        cur_trans->flags = 0;
        cur_trans->start_time = ktime_get_seconds();

        memset(&cur_trans->delayed_refs, 0, sizeof(cur_trans->delayed_refs));

        cur_trans->delayed_refs.href_root = RB_ROOT_CACHED;
        cur_trans->delayed_refs.dirty_extent_root = RB_ROOT;
        atomic_set(&cur_trans->delayed_refs.num_entries, 0);

        /*
         * although the tree mod log is per file system and not per transaction,
         * the log must never go across transaction boundaries.
         */
        smp_mb();
        if (!list_empty(&fs_info->tree_mod_seq_list))
                WARN(1, KERN_ERR "BTRFS: tree_mod_seq_list not empty when creating a fresh transaction\n");
        if (!RB_EMPTY_ROOT(&fs_info->tree_mod_log))
                WARN(1, KERN_ERR "BTRFS: tree_mod_log rb tree not empty when creating a fresh transaction\n");
        atomic64_set(&fs_info->tree_mod_seq, 0);

        spin_lock_init(&cur_trans->delayed_refs.lock);

        INIT_LIST_HEAD(&cur_trans->pending_snapshots);
        INIT_LIST_HEAD(&cur_trans->dev_update_list);
        INIT_LIST_HEAD(&cur_trans->switch_commits);
        INIT_LIST_HEAD(&cur_trans->dirty_bgs);
        INIT_LIST_HEAD(&cur_trans->io_bgs);
        INIT_LIST_HEAD(&cur_trans->dropped_roots);
        mutex_init(&cur_trans->cache_write_mutex);
        spin_lock_init(&cur_trans->dirty_bgs_lock);
        INIT_LIST_HEAD(&cur_trans->deleted_bgs);
        spin_lock_init(&cur_trans->dropped_roots_lock);
        INIT_LIST_HEAD(&cur_trans->releasing_ebs);
        spin_lock_init(&cur_trans->releasing_ebs_lock);
        list_add_tail(&cur_trans->list, &fs_info->trans_list);
        extent_io_tree_init(fs_info, &cur_trans->dirty_pages,
                        IO_TREE_TRANS_DIRTY_PAGES, fs_info->btree_inode);
        extent_io_tree_init(fs_info, &cur_trans->pinned_extents,
                        IO_TREE_FS_PINNED_EXTENTS, NULL);
        fs_info->generation++;
        cur_trans->transid = fs_info->generation;
        fs_info->running_transaction = cur_trans;
        cur_trans->aborted = 0;
        spin_unlock(&fs_info->trans_lock);

        return 0;
}

/*
 * This does all the record keeping required to make sure that a shareable root
 * is properly recorded in a given transaction.  This is required to make sure
 * the old root from before we joined the transaction is deleted when the
 * transaction commits.
 */
static int record_root_in_trans(struct btrfs_trans_handle *trans,
                               struct btrfs_root *root,
                               int force)
{
        struct btrfs_fs_info *fs_info = root->fs_info;
        int ret = 0;

        if ((test_bit(BTRFS_ROOT_SHAREABLE, &root->state) &&
            root->last_trans < trans->transid) || force) {
                WARN_ON(root == fs_info->extent_root);
                WARN_ON(!force && root->commit_root != root->node);

                /*
                 * see below for IN_TRANS_SETUP usage rules
                 * we have the reloc mutex held now, so there
                 * is only one writer in this function
                 */
                set_bit(BTRFS_ROOT_IN_TRANS_SETUP, &root->state);

                /* make sure readers find IN_TRANS_SETUP before
                 * they find our root->last_trans update
                 */
                smp_wmb();

                spin_lock(&fs_info->fs_roots_radix_lock);
                if (root->last_trans == trans->transid && !force) {
                        spin_unlock(&fs_info->fs_roots_radix_lock);
                        return 0;
                }
                radix_tree_tag_set(&fs_info->fs_roots_radix,
                                   (unsigned long)root->root_key.objectid,
                                   BTRFS_ROOT_TRANS_TAG);
                spin_unlock(&fs_info->fs_roots_radix_lock);
                root->last_trans = trans->transid;

                /* this is pretty tricky.  We don't want to
                 * take the relocation lock in btrfs_record_root_in_trans
                 * unless we're really doing the first setup for this root in
                 * this transaction.
                 *
                 * Normally we'd use root->last_trans as a flag to decide
                 * if we want to take the expensive mutex.
                 *
                 * But, we have to set root->last_trans before we
                 * init the relocation root, otherwise, we trip over warnings
                 * in ctree.c.  The solution used here is to flag ourselves
                 * with root IN_TRANS_SETUP.  When this is 1, we're still
                 * fixing up the reloc trees and everyone must wait.
                 *
                 * When this is zero, they can trust root->last_trans and fly
                 * through btrfs_record_root_in_trans without having to take the
                 * lock.  smp_wmb() makes sure that all the writes above are
                 * done before we pop in the zero below
                 */
                ret = btrfs_init_reloc_root(trans, root);
                smp_mb__before_atomic();
                clear_bit(BTRFS_ROOT_IN_TRANS_SETUP, &root->state);
        }
        return ret;
}


void btrfs_add_dropped_root(struct btrfs_trans_handle *trans,
                            struct btrfs_root *root)
{
        struct btrfs_fs_info *fs_info = root->fs_info;
        struct btrfs_transaction *cur_trans = trans->transaction;

        /* Add ourselves to the transaction dropped list */
        spin_lock(&cur_trans->dropped_roots_lock);
        list_add_tail(&root->root_list, &cur_trans->dropped_roots);
        spin_unlock(&cur_trans->dropped_roots_lock);

        /* Make sure we don't try to update the root at commit time */
        spin_lock(&fs_info->fs_roots_radix_lock);
        radix_tree_tag_clear(&fs_info->fs_roots_radix,
                             (unsigned long)root->root_key.objectid,
                             BTRFS_ROOT_TRANS_TAG);
        spin_unlock(&fs_info->fs_roots_radix_lock);
}

int btrfs_record_root_in_trans(struct btrfs_trans_handle *trans,
                               struct btrfs_root *root)
{
        struct btrfs_fs_info *fs_info = root->fs_info;
        int ret;

        if (!test_bit(BTRFS_ROOT_SHAREABLE, &root->state))
                return 0;

        /*
         * see record_root_in_trans for comments about IN_TRANS_SETUP usage
         * and barriers
         */
        smp_rmb();
        if (root->last_trans == trans->transid &&
            !test_bit(BTRFS_ROOT_IN_TRANS_SETUP, &root->state))
                return 0;

        mutex_lock(&fs_info->reloc_mutex);
        ret = record_root_in_trans(trans, root, 0);
        mutex_unlock(&fs_info->reloc_mutex);

        return ret;
}

static inline int is_transaction_blocked(struct btrfs_transaction *trans)
{
        return (trans->state >= TRANS_STATE_COMMIT_START &&
                trans->state < TRANS_STATE_UNBLOCKED &&
                !TRANS_ABORTED(trans));
}

/* wait for commit against the current transaction to become unblocked
 * when this is done, it is safe to start a new transaction, but the current
 * transaction might not be fully on disk.
 */
static void wait_current_trans(struct btrfs_fs_info *fs_info)
{
        struct btrfs_transaction *cur_trans;

        spin_lock(&fs_info->trans_lock);
        cur_trans = fs_info->running_transaction;
        if (cur_trans && is_transaction_blocked(cur_trans)) {
                refcount_inc(&cur_trans->use_count);
                spin_unlock(&fs_info->trans_lock);

                wait_event(fs_info->transaction_wait,
                           cur_trans->state >= TRANS_STATE_UNBLOCKED ||
                           TRANS_ABORTED(cur_trans));
                btrfs_put_transaction(cur_trans);
        } else {
                spin_unlock(&fs_info->trans_lock);
        }
}

static int may_wait_transaction(struct btrfs_fs_info *fs_info, int type)
{
        if (test_bit(BTRFS_FS_LOG_RECOVERING, &fs_info->flags))
                return 0;

        if (type == TRANS_START)
                return 1;

        return 0;
}

static inline bool need_reserve_reloc_root(struct btrfs_root *root)
{
        struct btrfs_fs_info *fs_info = root->fs_info;

        if (!fs_info->reloc_ctl ||
            !test_bit(BTRFS_ROOT_SHAREABLE, &root->state) ||
            root->root_key.objectid == BTRFS_TREE_RELOC_OBJECTID ||
            root->reloc_root)
                return false;

        return true;
}

static struct btrfs_trans_handle *
start_transaction(struct btrfs_root *root, unsigned int num_items,
                  unsigned int type, enum btrfs_reserve_flush_enum flush,
                  bool enforce_qgroups)
{
        struct btrfs_fs_info *fs_info = root->fs_info;
        struct btrfs_block_rsv *delayed_refs_rsv = &fs_info->delayed_refs_rsv;
        struct btrfs_trans_handle *h;
        struct btrfs_transaction *cur_trans;
        u64 num_bytes = 0;
        u64 qgroup_reserved = 0;
        bool reloc_reserved = false;
        bool do_chunk_alloc = false;
        int ret;

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

        if (current->journal_info) {
                WARN_ON(type & TRANS_EXTWRITERS);
                h = current->journal_info;
                refcount_inc(&h->use_count);
                WARN_ON(refcount_read(&h->use_count) > 2);
                h->orig_rsv = h->block_rsv;
                h->block_rsv = NULL;
                goto got_it;
        }

        /*
         * Do the reservation before we join the transaction so we can do all
         * the appropriate flushing if need be.
         */
        if (num_items && root != fs_info->chunk_root) {
                struct btrfs_block_rsv *rsv = &fs_info->trans_block_rsv;
                u64 delayed_refs_bytes = 0;

                qgroup_reserved = num_items * fs_info->nodesize;
                ret = btrfs_qgroup_reserve_meta_pertrans(root, qgroup_reserved,
                                enforce_qgroups);
                if (ret)
                        return ERR_PTR(ret);

                /*
                 * We want to reserve all the bytes we may need all at once, so
                 * we only do 1 enospc flushing cycle per transaction start.  We
                 * accomplish this by simply assuming we'll do 2 x num_items
                 * worth of delayed refs updates in this trans handle, and
                 * refill that amount for whatever is missing in the reserve.
                 */
                num_bytes = btrfs_calc_insert_metadata_size(fs_info, num_items);
                if (flush == BTRFS_RESERVE_FLUSH_ALL &&
                    delayed_refs_rsv->full == 0) {
                        delayed_refs_bytes = num_bytes;
                        num_bytes <<= 1;
                }

                /*
                 * Do the reservation for the relocation root creation
                 */
                if (need_reserve_reloc_root(root)) {
                        num_bytes += fs_info->nodesize;
                        reloc_reserved = true;
                }

                ret = btrfs_block_rsv_add(root, rsv, num_bytes, flush);
                if (ret)
                        goto reserve_fail;
                if (delayed_refs_bytes) {
                        btrfs_migrate_to_delayed_refs_rsv(fs_info, rsv,
                                                          delayed_refs_bytes);
                        num_bytes -= delayed_refs_bytes;
                }

                if (rsv->space_info->force_alloc)
                        do_chunk_alloc = true;
        } else if (num_items == 0 && flush == BTRFS_RESERVE_FLUSH_ALL &&
                   !delayed_refs_rsv->full) {
                /*
                 * Some people call with btrfs_start_transaction(root, 0)
                 * because they can be throttled, but have some other mechanism
                 * for reserving space.  We still want these guys to refill the
                 * delayed block_rsv so just add 1 items worth of reservation
                 * here.
                 */
                ret = btrfs_delayed_refs_rsv_refill(fs_info, flush);
                if (ret)
                        goto reserve_fail;
        }
again:
        h = kmem_cache_zalloc(btrfs_trans_handle_cachep, GFP_NOFS);
        if (!h) {
                ret = -ENOMEM;
                goto alloc_fail;
        }

        /*
         * If we are JOIN_NOLOCK we're already committing a transaction and
         * waiting on this guy, so we don't need to do the sb_start_intwrite
         * because we're already holding a ref.  We need this because we could
         * have raced in and did an fsync() on a file which can kick a commit
         * and then we deadlock with somebody doing a freeze.
         *
         * If we are ATTACH, it means we just want to catch the current
         * transaction and commit it, so we needn't do sb_start_intwrite(). 
         */
        if (type & __TRANS_FREEZABLE)
                sb_start_intwrite(fs_info->sb);

        if (may_wait_transaction(fs_info, type))
                wait_current_trans(fs_info);

        do {
                ret = join_transaction(fs_info, type);
                if (ret == -EBUSY) {
                        wait_current_trans(fs_info);
                        if (unlikely(type == TRANS_ATTACH ||
                                     type == TRANS_JOIN_NOSTART))
                                ret = -ENOENT;
                }
        } while (ret == -EBUSY);

        if (ret < 0)
                goto join_fail;

        cur_trans = fs_info->running_transaction;

        h->transid = cur_trans->transid;
        h->transaction = cur_trans;
        h->root = root;
        refcount_set(&h->use_count, 1);
        h->fs_info = root->fs_info;

        h->type = type;
        INIT_LIST_HEAD(&h->new_bgs);

        smp_mb();
        if (cur_trans->state >= TRANS_STATE_COMMIT_START &&
            may_wait_transaction(fs_info, type)) {
                current->journal_info = h;
                btrfs_commit_transaction(h);
                goto again;
        }

        if (num_bytes) {
                trace_btrfs_space_reservation(fs_info, "transaction",
                                              h->transid, num_bytes, 1);
                h->block_rsv = &fs_info->trans_block_rsv;
                h->bytes_reserved = num_bytes;
                h->reloc_reserved = reloc_reserved;
        }

got_it:
        if (!current->journal_info)
                current->journal_info = h;

        /*
         * If the space_info is marked ALLOC_FORCE then we'll get upgraded to
         * ALLOC_FORCE the first run through, and then we won't allocate for
         * anybody else who races in later.  We don't care about the return
         * value here.
         */
        if (do_chunk_alloc && num_bytes) {
                u64 flags = h->block_rsv->space_info->flags;

                btrfs_chunk_alloc(h, btrfs_get_alloc_profile(fs_info, flags),
                                  CHUNK_ALLOC_NO_FORCE);
        }

        /*
         * btrfs_record_root_in_trans() needs to alloc new extents, and may
         * call btrfs_join_transaction() while we're also starting a
         * transaction.
         *
         * Thus it need to be called after current->journal_info initialized,
         * or we can deadlock.
         */
        ret = btrfs_record_root_in_trans(h, root);
        if (ret) {
                /*
                 * The transaction handle is fully initialized and linked with
                 * other structures so it needs to be ended in case of errors,
                 * not just freed.
                 */
                btrfs_end_transaction(h);
                return ERR_PTR(ret);
        }

        return h;

join_fail:
        if (type & __TRANS_FREEZABLE)
                sb_end_intwrite(fs_info->sb);
        kmem_cache_free(btrfs_trans_handle_cachep, h);
alloc_fail:
        if (num_bytes)
                btrfs_block_rsv_release(fs_info, &fs_info->trans_block_rsv,
                                        num_bytes, NULL);
reserve_fail:
        btrfs_qgroup_free_meta_pertrans(root, qgroup_reserved);
        return ERR_PTR(ret);
}

struct btrfs_trans_handle *btrfs_start_transaction(struct btrfs_root *root,
                                                   unsigned int num_items)
{
        return start_transaction(root, num_items, TRANS_START,
                                 BTRFS_RESERVE_FLUSH_ALL, true);
}

struct btrfs_trans_handle *btrfs_start_transaction_fallback_global_rsv(
                                        struct btrfs_root *root,
                                        unsigned int num_items)
{
        return start_transaction(root, num_items, TRANS_START,
                                 BTRFS_RESERVE_FLUSH_ALL_STEAL, false);
}

struct btrfs_trans_handle *btrfs_join_transaction(struct btrfs_root *root)
{
        return start_transaction(root, 0, TRANS_JOIN, BTRFS_RESERVE_NO_FLUSH,
                                 true);
}

struct btrfs_trans_handle *btrfs_join_transaction_spacecache(struct btrfs_root *root)
{
        return start_transaction(root, 0, TRANS_JOIN_NOLOCK,
                                 BTRFS_RESERVE_NO_FLUSH, true);
}

/*
 * Similar to regular join but it never starts a transaction when none is
 * running or after waiting for the current one to finish.
 */
struct btrfs_trans_handle *btrfs_join_transaction_nostart(struct btrfs_root *root)
{
        return start_transaction(root, 0, TRANS_JOIN_NOSTART,
                                 BTRFS_RESERVE_NO_FLUSH, true);
}

/*
 * btrfs_attach_transaction() - catch the running transaction
 *
 * It is used when we want to commit the current the transaction, but
 * don't want to start a new one.
 *
 * Note: If this function return -ENOENT, it just means there is no
 * running transaction. But it is possible that the inactive transaction
 * is still in the memory, not fully on disk. If you hope there is no
 * inactive transaction in the fs when -ENOENT is returned, you should
 * invoke
 *     btrfs_attach_transaction_barrier()
 */
struct btrfs_trans_handle *btrfs_attach_transaction(struct btrfs_root *root)
{
        return start_transaction(root, 0, TRANS_ATTACH,
                                 BTRFS_RESERVE_NO_FLUSH, true);
}

/*
 * btrfs_attach_transaction_barrier() - catch the running transaction
 *
 * It is similar to the above function, the difference is this one
 * will wait for all the inactive transactions until they fully
 * complete.
 */
struct btrfs_trans_handle *
btrfs_attach_transaction_barrier(struct btrfs_root *root)
{
        struct btrfs_trans_handle *trans;

        trans = start_transaction(root, 0, TRANS_ATTACH,
                                  BTRFS_RESERVE_NO_FLUSH, true);
        if (trans == ERR_PTR(-ENOENT))
                btrfs_wait_for_commit(root->fs_info, 0);

        return trans;
}

/* Wait for a transaction commit to reach at least the given state. */
static noinline void wait_for_commit(struct btrfs_transaction *commit,
                                     const enum btrfs_trans_state min_state)
{
        struct btrfs_fs_info *fs_info = commit->fs_info;
        u64 transid = commit->transid;
        bool put = false;

        while (1) {
                wait_event(commit->commit_wait, commit->state >= min_state);
                if (put)
                        btrfs_put_transaction(commit);

                if (min_state < TRANS_STATE_COMPLETED)
                        break;

                /*
                 * A transaction isn't really completed until all of the
                 * previous transactions are completed, but with fsync we can
                 * end up with SUPER_COMMITTED transactions before a COMPLETED
                 * transaction. Wait for those.
                 */

                spin_lock(&fs_info->trans_lock);
                commit = list_first_entry_or_null(&fs_info->trans_list,
                                                  struct btrfs_transaction,
                                                  list);
                if (!commit || commit->transid > transid) {
                        spin_unlock(&fs_info->trans_lock);
                        break;
                }
                refcount_inc(&commit->use_count);
                put = true;
                spin_unlock(&fs_info->trans_lock);
        }
}

int btrfs_wait_for_commit(struct btrfs_fs_info *fs_info, u64 transid)
{
        struct btrfs_transaction *cur_trans = NULL, *t;
        int ret = 0;

        if (transid) {
                if (transid <= fs_info->last_trans_committed)
                        goto out;

                /* find specified transaction */
                spin_lock(&fs_info->trans_lock);
                list_for_each_entry(t, &fs_info->trans_list, list) {
                        if (t->transid == transid) {
                                cur_trans = t;
                                refcount_inc(&cur_trans->use_count);
                                ret = 0;
                                break;
                        }
                        if (t->transid > transid) {
                                ret = 0;
                                break;
                        }
                }
                spin_unlock(&fs_info->trans_lock);

                /*
                 * The specified transaction doesn't exist, or we
                 * raced with btrfs_commit_transaction
                 */
                if (!cur_trans) {
                        if (transid > fs_info->last_trans_committed)
                                ret = -EINVAL;
                        goto out;
                }
        } else {
                /* find newest transaction that is committing | committed */
                spin_lock(&fs_info->trans_lock);
                list_for_each_entry_reverse(t, &fs_info->trans_list,
                                            list) {
                        if (t->state >= TRANS_STATE_COMMIT_START) {
                                if (t->state == TRANS_STATE_COMPLETED)
                                        break;
                                cur_trans = t;
                                refcount_inc(&cur_trans->use_count);
                                break;
                        }
                }
                spin_unlock(&fs_info->trans_lock);
                if (!cur_trans)
                        goto out;  /* nothing committing|committed */
        }

        wait_for_commit(cur_trans, TRANS_STATE_COMPLETED);
        btrfs_put_transaction(cur_trans);
out:
        return ret;
}

void btrfs_throttle(struct btrfs_fs_info *fs_info)
{
        wait_current_trans(fs_info);
}

static bool should_end_transaction(struct btrfs_trans_handle *trans)
{
        struct btrfs_fs_info *fs_info = trans->fs_info;

        if (btrfs_check_space_for_delayed_refs(fs_info))
                return true;

        return !!btrfs_block_rsv_check(&fs_info->global_block_rsv, 5);
}

bool btrfs_should_end_transaction(struct btrfs_trans_handle *trans)
{
        struct btrfs_transaction *cur_trans = trans->transaction;

        if (cur_trans->state >= TRANS_STATE_COMMIT_START ||
            test_bit(BTRFS_DELAYED_REFS_FLUSHING, &cur_trans->delayed_refs.flags))
                return true;

        return should_end_transaction(trans);
}

static void btrfs_trans_release_metadata(struct btrfs_trans_handle *trans)

{
        struct btrfs_fs_info *fs_info = trans->fs_info;

        if (!trans->block_rsv) {
                ASSERT(!trans->bytes_reserved);
                return;
        }

        if (!trans->bytes_reserved)
                return;

        ASSERT(trans->block_rsv == &fs_info->trans_block_rsv);
        trace_btrfs_space_reservation(fs_info, "transaction",
                                      trans->transid, trans->bytes_reserved, 0);
        btrfs_block_rsv_release(fs_info, trans->block_rsv,
                                trans->bytes_reserved, NULL);
        trans->bytes_reserved = 0;
}

static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
                                   int throttle)
{
        struct btrfs_fs_info *info = trans->fs_info;
        struct btrfs_transaction *cur_trans = trans->transaction;
        int err = 0;

        if (refcount_read(&trans->use_count) > 1) {
                refcount_dec(&trans->use_count);
                trans->block_rsv = trans->orig_rsv;
                return 0;
        }

        btrfs_trans_release_metadata(trans);
        trans->block_rsv = NULL;

        btrfs_create_pending_block_groups(trans);

        btrfs_trans_release_chunk_metadata(trans);

        if (trans->type & __TRANS_FREEZABLE)
                sb_end_intwrite(info->sb);

        WARN_ON(cur_trans != info->running_transaction);
        WARN_ON(atomic_read(&cur_trans->num_writers) < 1);
        atomic_dec(&cur_trans->num_writers);
        extwriter_counter_dec(cur_trans, trans->type);

        cond_wake_up(&cur_trans->writer_wait);
        btrfs_put_transaction(cur_trans);

        if (current->journal_info == trans)
                current->journal_info = NULL;

        if (throttle)
                btrfs_run_delayed_iputs(info);

        if (TRANS_ABORTED(trans) ||
            test_bit(BTRFS_FS_STATE_ERROR, &info->fs_state)) {
                wake_up_process(info->transaction_kthread);
                if (TRANS_ABORTED(trans))
                        err = trans->aborted;
                else
                        err = -EROFS;
        }

        kmem_cache_free(btrfs_trans_handle_cachep, trans);
        return err;
}

int btrfs_end_transaction(struct btrfs_trans_handle *trans)
{
        return __btrfs_end_transaction(trans, 0);
}

int btrfs_end_transaction_throttle(struct btrfs_trans_handle *trans)
{
        return __btrfs_end_transaction(trans, 1);
}

/*
 * when btree blocks are allocated, they have some corresponding bits set for
 * them in one of two extent_io trees.  This is used to make sure all of
 * those extents are sent to disk but does not wait on them
 */
int btrfs_write_marked_extents(struct btrfs_fs_info *fs_info,
                               struct extent_io_tree *dirty_pages, int mark)
{
        int err = 0;
        int werr = 0;
        struct address_space *mapping = fs_info->btree_inode->i_mapping;
        struct extent_state *cached_state = NULL;
        u64 start = 0;
        u64 end;

        atomic_inc(&BTRFS_I(fs_info->btree_inode)->sync_writers);
        while (!find_first_extent_bit(dirty_pages, start, &start, &end,
                                      mark, &cached_state)) {
                bool wait_writeback = false;

                err = convert_extent_bit(dirty_pages, start, end,
                                         EXTENT_NEED_WAIT,
                                         mark, &cached_state);
                /*
                 * convert_extent_bit can return -ENOMEM, which is most of the
                 * time a temporary error. So when it happens, ignore the error
                 * and wait for writeback of this range to finish - because we
                 * failed to set the bit EXTENT_NEED_WAIT for the range, a call
                 * to __btrfs_wait_marked_extents() would not know that
                 * writeback for this range started and therefore wouldn't
                 * wait for it to finish - we don't want to commit a
                 * superblock that points to btree nodes/leafs for which
                 * writeback hasn't finished yet (and without errors).
                 * We cleanup any entries left in the io tree when committing
                 * the transaction (through extent_io_tree_release()).
                 */
                if (err == -ENOMEM) {
                        err = 0;
                        wait_writeback = true;
                }
                if (!err)
                        err = filemap_fdatawrite_range(mapping, start, end);
                if (err)
                        werr = err;
                else if (wait_writeback)
                        werr = filemap_fdatawait_range(mapping, start, end);
                free_extent_state(cached_state);
                cached_state = NULL;
                cond_resched();
                start = end + 1;
        }
        atomic_dec(&BTRFS_I(fs_info->btree_inode)->sync_writers);
        return werr;
}

/*
 * when btree blocks are allocated, they have some corresponding bits set for
 * them in one of two extent_io trees.  This is used to make sure all of
 * those extents are on disk for transaction or log commit.  We wait
 * on all the pages and clear them from the dirty pages state tree
 */
static int __btrfs_wait_marked_extents(struct btrfs_fs_info *fs_info,
                                       struct extent_io_tree *dirty_pages)
{
        int err = 0;
        int werr = 0;
        struct address_space *mapping = fs_info->btree_inode->i_mapping;
        struct extent_state *cached_state = NULL;
        u64 start = 0;
        u64 end;

        while (!find_first_extent_bit(dirty_pages, start, &start, &end,
                                      EXTENT_NEED_WAIT, &cached_state)) {
                /*
                 * Ignore -ENOMEM errors returned by clear_extent_bit().
                 * When committing the transaction, we'll remove any entries
                 * left in the io tree. For a log commit, we don't remove them
                 * after committing the log because the tree can be accessed
                 * concurrently - we do it only at transaction commit time when
                 * it's safe to do it (through extent_io_tree_release()).
                 */
                err = clear_extent_bit(dirty_pages, start, end,
                                       EXTENT_NEED_WAIT, 0, 0, &cached_state);
                if (err == -ENOMEM)
                        err = 0;
                if (!err)
                        err = filemap_fdatawait_range(mapping, start, end);
                if (err)
                        werr = err;
                free_extent_state(cached_state);
                cached_state = NULL;
                cond_resched();
                start = end + 1;
        }
        if (err)
                werr = err;
        return werr;
}

static int btrfs_wait_extents(struct btrfs_fs_info *fs_info,
                       struct extent_io_tree *dirty_pages)
{
        bool errors = false;
        int err;

        err = __btrfs_wait_marked_extents(fs_info, dirty_pages);
        if (test_and_clear_bit(BTRFS_FS_BTREE_ERR, &fs_info->flags))
                errors = true;

        if (errors && !err)
                err = -EIO;
        return err;
}

int btrfs_wait_tree_log_extents(struct btrfs_root *log_root, int mark)
{
        struct btrfs_fs_info *fs_info = log_root->fs_info;
        struct extent_io_tree *dirty_pages = &log_root->dirty_log_pages;
        bool errors = false;
        int err;

        ASSERT(log_root->root_key.objectid == BTRFS_TREE_LOG_OBJECTID);

        err = __btrfs_wait_marked_extents(fs_info, dirty_pages);
        if ((mark & EXTENT_DIRTY) &&
            test_and_clear_bit(BTRFS_FS_LOG1_ERR, &fs_info->flags))
                errors = true;

        if ((mark & EXTENT_NEW) &&
            test_and_clear_bit(BTRFS_FS_LOG2_ERR, &fs_info->flags))
                errors = true;

        if (errors && !err)
                err = -EIO;
        return err;
}

/*
 * When btree blocks are allocated the corresponding extents are marked dirty.
 * This function ensures such extents are persisted on disk for transaction or
 * log commit.
 *
 * @trans: transaction whose dirty pages we'd like to write
 */
static int btrfs_write_and_wait_transaction(struct btrfs_trans_handle *trans)
{
        int ret;
        int ret2;
        struct extent_io_tree *dirty_pages = &trans->transaction->dirty_pages;
        struct btrfs_fs_info *fs_info = trans->fs_info;
        struct blk_plug plug;

        blk_start_plug(&plug);
        ret = btrfs_write_marked_extents(fs_info, dirty_pages, EXTENT_DIRTY);
        blk_finish_plug(&plug);
        ret2 = btrfs_wait_extents(fs_info, dirty_pages);

        extent_io_tree_release(&trans->transaction->dirty_pages);

        if (ret)
                return ret;
        else if (ret2)
                return ret2;
        else
                return 0;
}

/*
 * this is used to update the root pointer in the tree of tree roots.
 *
 * But, in the case of the extent allocation tree, updating the root
 * pointer may allocate blocks which may change the root of the extent
 * allocation tree.
 *
 * So, this loops and repeats and makes sure the cowonly root didn't
 * change while the root pointer was being updated in the metadata.
 */
static int update_cowonly_root(struct btrfs_trans_handle *trans,
                               struct btrfs_root *root)
{
        int ret;
        u64 old_root_bytenr;
        u64 old_root_used;
        struct btrfs_fs_info *fs_info = root->fs_info;
        struct btrfs_root *tree_root = fs_info->tree_root;

        old_root_used = btrfs_root_used(&root->root_item);

        while (1) {
                old_root_bytenr = btrfs_root_bytenr(&root->root_item);
                if (old_root_bytenr == root->node->start &&
                    old_root_used == btrfs_root_used(&root->root_item))
                        break;

                btrfs_set_root_node(&root->root_item, root->node);
                ret = btrfs_update_root(trans, tree_root,
                                        &root->root_key,
                                        &root->root_item);
                if (ret)
                        return ret;

                old_root_used = btrfs_root_used(&root->root_item);
        }

        return 0;
}

/*
 * update all the cowonly tree roots on disk
 *
 * The error handling in this function may not be obvious. Any of the
 * failures will cause the file system to go offline. We still need
 * to clean up the delayed refs.
 */
static noinline int commit_cowonly_roots(struct btrfs_trans_handle *trans)
{
        struct btrfs_fs_info *fs_info = trans->fs_info;
        struct list_head *dirty_bgs = &trans->transaction->dirty_bgs;
        struct list_head *io_bgs = &trans->transaction->io_bgs;
        struct list_head *next;
        struct extent_buffer *eb;
        int ret;

        eb = btrfs_lock_root_node(fs_info->tree_root);
        ret = btrfs_cow_block(trans, fs_info->tree_root, eb, NULL,
                              0, &eb, BTRFS_NESTING_COW);
        btrfs_tree_unlock(eb);
        free_extent_buffer(eb);

        if (ret)
                return ret;

        ret = btrfs_run_dev_stats(trans);
        if (ret)
                return ret;
        ret = btrfs_run_dev_replace(trans);
        if (ret)
                return ret;
        ret = btrfs_run_qgroups(trans);
        if (ret)
                return ret;

        ret = btrfs_setup_space_cache(trans);
        if (ret)
                return ret;

again:
        while (!list_empty(&fs_info->dirty_cowonly_roots)) {
                struct btrfs_root *root;
                next = fs_info->dirty_cowonly_roots.next;
                list_del_init(next);
                root = list_entry(next, struct btrfs_root, dirty_list);
                clear_bit(BTRFS_ROOT_DIRTY, &root->state);

                if (root != fs_info->extent_root)
                        list_add_tail(&root->dirty_list,
                                      &trans->transaction->switch_commits);
                ret = update_cowonly_root(trans, root);
                if (ret)
                        return ret;
        }

        /* Now flush any delayed refs generated by updating all of the roots */
        ret = btrfs_run_delayed_refs(trans, (unsigned long)-1);
        if (ret)
                return ret;

        while (!list_empty(dirty_bgs) || !list_empty(io_bgs)) {
                ret = btrfs_write_dirty_block_groups(trans);
                if (ret)
                        return ret;

                /*
                 * We're writing the dirty block groups, which could generate
                 * delayed refs, which could generate more dirty block groups,
                 * so we want to keep this flushing in this loop to make sure
                 * everything gets run.
                 */
                ret = btrfs_run_delayed_refs(trans, (unsigned long)-1);
                if (ret)
                        return ret;
        }

        if (!list_empty(&fs_info->dirty_cowonly_roots))
                goto again;

        list_add_tail(&fs_info->extent_root->dirty_list,
                      &trans->transaction->switch_commits);

        /* Update dev-replace pointer once everything is committed */
        fs_info->dev_replace.committed_cursor_left =
                fs_info->dev_replace.cursor_left_last_write_of_item;

        return 0;
}

/*
 * If we had a pending drop we need to see if there are any others left in our
 * dead roots list, and if not clear our bit and wake any waiters.
 */
void btrfs_maybe_wake_unfinished_drop(struct btrfs_fs_info *fs_info)
{
        /*
         * We put the drop in progress roots at the front of the list, so if the
         * first entry doesn't have UNFINISHED_DROP set we can wake everybody
         * up.
         */
        spin_lock(&fs_info->trans_lock);
        if (!list_empty(&fs_info->dead_roots)) {
                struct btrfs_root *root = list_first_entry(&fs_info->dead_roots,
                                                           struct btrfs_root,
                                                           root_list);
                if (test_bit(BTRFS_ROOT_UNFINISHED_DROP, &root->state)) {
                        spin_unlock(&fs_info->trans_lock);
                        return;
                }
        }
        spin_unlock(&fs_info->trans_lock);

        btrfs_wake_unfinished_drop(fs_info);
}

/*
 * dead roots are old snapshots that need to be deleted.  This allocates
 * a dirty root struct and adds it into the list of dead roots that need to
 * be deleted
 */
void btrfs_add_dead_root(struct btrfs_root *root)
{
        struct btrfs_fs_info *fs_info = root->fs_info;

        spin_lock(&fs_info->trans_lock);
        if (list_empty(&root->root_list)) {
                btrfs_grab_root(root);

                /* We want to process the partially complete drops first. */
                if (test_bit(BTRFS_ROOT_UNFINISHED_DROP, &root->state))
                        list_add(&root->root_list, &fs_info->dead_roots);
                else
                        list_add_tail(&root->root_list, &fs_info->dead_roots);
        }
        spin_unlock(&fs_info->trans_lock);
}

/*
 * update all the cowonly tree roots on disk
 */
static noinline int commit_fs_roots(struct btrfs_trans_handle *trans)
{
        struct btrfs_fs_info *fs_info = trans->fs_info;
        struct btrfs_root *gang[8];
        int i;
        int ret;

        spin_lock(&fs_info->fs_roots_radix_lock);
        while (1) {
                ret = radix_tree_gang_lookup_tag(&fs_info->fs_roots_radix,
                                                 (void **)gang, 0,
                                                 ARRAY_SIZE(gang),
                                                 BTRFS_ROOT_TRANS_TAG);
                if (ret == 0)
                        break;
                for (i = 0; i < ret; i++) {
                        struct btrfs_root *root = gang[i];
                        int ret2;

                        radix_tree_tag_clear(&fs_info->fs_roots_radix,
                                        (unsigned long)root->root_key.objectid,
                                        BTRFS_ROOT_TRANS_TAG);
                        spin_unlock(&fs_info->fs_roots_radix_lock);

                        btrfs_free_log(trans, root);
                        ret2 = btrfs_update_reloc_root(trans, root);
                        if (ret2)
                                return ret2;

                        /* see comments in should_cow_block() */
                        clear_bit(BTRFS_ROOT_FORCE_COW, &root->state);
                        smp_mb__after_atomic();

                        if (root->commit_root != root->node) {
                                list_add_tail(&root->dirty_list,
                                        &trans->transaction->switch_commits);
                                btrfs_set_root_node(&root->root_item,
                                                    root->node);
                        }

                        ret2 = btrfs_update_root(trans, fs_info->tree_root,
                                                &root->root_key,
                                                &root->root_item);
                        if (ret2)
                                return ret2;
                        spin_lock(&fs_info->fs_roots_radix_lock);
                        btrfs_qgroup_free_meta_all_pertrans(root);
                }
        }
        spin_unlock(&fs_info->fs_roots_radix_lock);
        return 0;
}

/*
 * defrag a given btree.
 * Every leaf in the btree is read and defragged.
 */
int btrfs_defrag_root(struct btrfs_root *root)
{
        struct btrfs_fs_info *info = root->fs_info;
        struct btrfs_trans_handle *trans;
        int ret;

        if (test_and_set_bit(BTRFS_ROOT_DEFRAG_RUNNING, &root->state))
                return 0;

        while (1) {
                trans = btrfs_start_transaction(root, 0);
                if (IS_ERR(trans)) {
                        ret = PTR_ERR(trans);
                        break;
                }

                ret = btrfs_defrag_leaves(trans, root);

                btrfs_end_transaction(trans);
                btrfs_btree_balance_dirty(info);
                cond_resched();

                if (btrfs_fs_closing(info) || ret != -EAGAIN)
                        break;

                if (btrfs_defrag_cancelled(info)) {
                        btrfs_debug(info, "defrag_root cancelled");
                        ret = -EAGAIN;
                        break;
                }
        }
        clear_bit(BTRFS_ROOT_DEFRAG_RUNNING, &root->state);
        return ret;
}

/*
 * Do all special snapshot related qgroup dirty hack.
 *
 * Will do all needed qgroup inherit and dirty hack like switch commit
 * roots inside one transaction and write all btree into disk, to make
 * qgroup works.
 */
static int qgroup_account_snapshot(struct btrfs_trans_handle *trans,
                                   struct btrfs_root *src,
                                   struct btrfs_root *parent,
                                   struct btrfs_qgroup_inherit *inherit,
                                   u64 dst_objectid)
{
        struct btrfs_fs_info *fs_info = src->fs_info;
        int ret;

        /*
         * Save some performance in the case that qgroups are not
         * enabled. If this check races with the ioctl, rescan will
         * kick in anyway.
         */
        if (!test_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags))
                return 0;

        /*
         * Ensure dirty @src will be committed.  Or, after coming
         * commit_fs_roots() and switch_commit_roots(), any dirty but not
         * recorded root will never be updated again, causing an outdated root
         * item.
         */
        ret = record_root_in_trans(trans, src, 1);
        if (ret)
                return ret;

        /*
         * btrfs_qgroup_inherit relies on a consistent view of the usage for the
         * src root, so we must run the delayed refs here.
         *
         * However this isn't particularly fool proof, because there's no
         * synchronization keeping us from changing the tree after this point
         * before we do the qgroup_inherit, or even from making changes while
         * we're doing the qgroup_inherit.  But that's a problem for the future,
         * for now flush the delayed refs to narrow the race window where the
         * qgroup counters could end up wrong.
         */
        ret = btrfs_run_delayed_refs(trans, (unsigned long)-1);
        if (ret) {
                btrfs_abort_transaction(trans, ret);
                return ret;
        }

        /*
         * We are going to commit transaction, see btrfs_commit_transaction()
         * comment for reason locking tree_log_mutex
         */
        mutex_lock(&fs_info->tree_log_mutex);

        ret = commit_fs_roots(trans);
        if (ret)
                goto out;
        ret = btrfs_qgroup_account_extents(trans);
        if (ret < 0)
                goto out;

        /* Now qgroup are all updated, we can inherit it to new qgroups */
        ret = btrfs_qgroup_inherit(trans, src->root_key.objectid, dst_objectid,
                                   inherit);
        if (ret < 0)
                goto out;

        /*
         * Now we do a simplified commit transaction, which will:
         * 1) commit all subvolume and extent tree
         *    To ensure all subvolume and extent tree have a valid
         *    commit_root to accounting later insert_dir_item()
         * 2) write all btree blocks onto disk
         *    This is to make sure later btree modification will be cowed
         *    Or commit_root can be populated and cause wrong qgroup numbers
         * In this simplified commit, we don't really care about other trees
         * like chunk and root tree, as they won't affect qgroup.
         * And we don't write super to avoid half committed status.
         */
        ret = commit_cowonly_roots(trans);
        if (ret)
                goto out;
        switch_commit_roots(trans);
        ret = btrfs_write_and_wait_transaction(trans);
        if (ret)
                btrfs_handle_fs_error(fs_info, ret,
                        "Error while writing out transaction for qgroup");

out:
        mutex_unlock(&fs_info->tree_log_mutex);

        /*
         * Force parent root to be updated, as we recorded it before so its
         * last_trans == cur_transid.
         * Or it won't be committed again onto disk after later
         * insert_dir_item()
         */
        if (!ret)
                ret = record_root_in_trans(trans, parent, 1);
        return ret;
}

/*
 * new snapshots need to be created at a very specific time in the
 * transaction commit.  This does the actual creation.
 *
 * Note:
 * If the error which may affect the commitment of the current transaction
 * happens, we should return the error number. If the error which just affect
 * the creation of the pending snapshots, just return 0.
 */
static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
                                   struct btrfs_pending_snapshot *pending)
{

        struct btrfs_fs_info *fs_info = trans->fs_info;
        struct btrfs_key key;
        struct btrfs_root_item *new_root_item;
        struct btrfs_root *tree_root = fs_info->tree_root;
        struct btrfs_root *root = pending->root;
        struct btrfs_root *parent_root;
        struct btrfs_block_rsv *rsv;
        struct inode *parent_inode;
        struct btrfs_path *path;
        struct btrfs_dir_item *dir_item;
        struct dentry *dentry;
        struct extent_buffer *tmp;
        struct extent_buffer *old;
        struct timespec64 cur_time;
        int ret = 0;
        u64 to_reserve = 0;
        u64 index = 0;
        u64 objectid;
        u64 root_flags;

        ASSERT(pending->path);
        path = pending->path;

        ASSERT(pending->root_item);
        new_root_item = pending->root_item;

        pending->error = btrfs_get_free_objectid(tree_root, &objectid);
        if (pending->error)
                goto no_free_objectid;

        /*
         * Make qgroup to skip current new snapshot's qgroupid, as it is
         * accounted by later btrfs_qgroup_inherit().
         */
        btrfs_set_skip_qgroup(trans, objectid);

        btrfs_reloc_pre_snapshot(pending, &to_reserve);

        if (to_reserve > 0) {
                pending->error = btrfs_block_rsv_add(root,
                                                     &pending->block_rsv,
                                                     to_reserve,
                                                     BTRFS_RESERVE_NO_FLUSH);
                if (pending->error)
                        goto clear_skip_qgroup;
        }

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

        rsv = trans->block_rsv;
        trans->block_rsv = &pending->block_rsv;
        trans->bytes_reserved = trans->block_rsv->reserved;
        trace_btrfs_space_reservation(fs_info, "transaction",
                                      trans->transid,
                                      trans->bytes_reserved, 1);
        dentry = pending->dentry;
        parent_inode = pending->dir;
        parent_root = BTRFS_I(parent_inode)->root;
        ret = record_root_in_trans(trans, parent_root, 0);
        if (ret)
                goto fail;
        cur_time = current_time(parent_inode);

        /*
         * insert the directory item
         */
        ret = btrfs_set_inode_index(BTRFS_I(parent_inode), &index);
        BUG_ON(ret); /* -ENOMEM */

        /* check if there is a file/dir which has the same name. */
        dir_item = btrfs_lookup_dir_item(NULL, parent_root, path,
                                         btrfs_ino(BTRFS_I(parent_inode)),
                                         dentry->d_name.name,
                                         dentry->d_name.len, 0);
        if (dir_item != NULL && !IS_ERR(dir_item)) {
                pending->error = -EEXIST;
                goto dir_item_existed;
        } else if (IS_ERR(dir_item)) {
                ret = PTR_ERR(dir_item);
                btrfs_abort_transaction(trans, ret);
                goto fail;
        }
        btrfs_release_path(path);

        /*
         * pull in the delayed directory update
         * and the delayed inode item
         * otherwise we corrupt the FS during
         * snapshot
         */
        ret = btrfs_run_delayed_items(trans);
        if (ret) {      /* Transaction aborted */
                btrfs_abort_transaction(trans, ret);
                goto fail;
        }

        ret = record_root_in_trans(trans, root, 0);
        if (ret) {
                btrfs_abort_transaction(trans, ret);
                goto fail;
        }
        btrfs_set_root_last_snapshot(&root->root_item, trans->transid);
        memcpy(new_root_item, &root->root_item, sizeof(*new_root_item));
        btrfs_check_and_init_root_item(new_root_item);

        root_flags = btrfs_root_flags(new_root_item);
        if (pending->readonly)
                root_flags |= BTRFS_ROOT_SUBVOL_RDONLY;
        else
                root_flags &= ~BTRFS_ROOT_SUBVOL_RDONLY;
        btrfs_set_root_flags(new_root_item, root_flags);

        btrfs_set_root_generation_v2(new_root_item,
                        trans->transid);
        generate_random_guid(new_root_item->uuid);
        memcpy(new_root_item->parent_uuid, root->root_item.uuid,
                        BTRFS_UUID_SIZE);
        if (!(root_flags & BTRFS_ROOT_SUBVOL_RDONLY)) {
                memset(new_root_item->received_uuid, 0,
                       sizeof(new_root_item->received_uuid));
                memset(&new_root_item->stime, 0, sizeof(new_root_item->stime));
                memset(&new_root_item->rtime, 0, sizeof(new_root_item->rtime));
                btrfs_set_root_stransid(new_root_item, 0);
                btrfs_set_root_rtransid(new_root_item, 0);
        }
        btrfs_set_stack_timespec_sec(&new_root_item->otime, cur_time.tv_sec);
        btrfs_set_stack_timespec_nsec(&new_root_item->otime, cur_time.tv_nsec);
        btrfs_set_root_otransid(new_root_item, trans->transid);

        old = btrfs_lock_root_node(root);
        ret = btrfs_cow_block(trans, root, old, NULL, 0, &old,
                              BTRFS_NESTING_COW);
        if (ret) {
                btrfs_tree_unlock(old);
                free_extent_buffer(old);
                btrfs_abort_transaction(trans, ret);
                goto fail;
        }

        ret = btrfs_copy_root(trans, root, old, &tmp, objectid);
        /* clean up in any case */
        btrfs_tree_unlock(old);
        free_extent_buffer(old);
        if (ret) {
                btrfs_abort_transaction(trans, ret);
                goto fail;
        }
        /* see comments in should_cow_block() */
        set_bit(BTRFS_ROOT_FORCE_COW, &root->state);
        smp_wmb();

        btrfs_set_root_node(new_root_item, tmp);
        /* record when the snapshot was created in key.offset */
        key.offset = trans->transid;
        ret = btrfs_insert_root(trans, tree_root, &key, new_root_item);
        btrfs_tree_unlock(tmp);
        free_extent_buffer(tmp);
        if (ret) {
                btrfs_abort_transaction(trans, ret);
                goto fail;
        }

        /*
         * insert root back/forward references
         */
        ret = btrfs_add_root_ref(trans, objectid,
                                 parent_root->root_key.objectid,
                                 btrfs_ino(BTRFS_I(parent_inode)), index,
                                 dentry->d_name.name, dentry->d_name.len);
        if (ret) {
                btrfs_abort_transaction(trans, ret);
                goto fail;
        }

        key.offset = (u64)-1;
        pending->snap = btrfs_get_new_fs_root(fs_info, objectid, pending->anon_dev);
        if (IS_ERR(pending->snap)) {
                ret = PTR_ERR(pending->snap);
                pending->snap = NULL;
                btrfs_abort_transaction(trans, ret);
                goto fail;
        }

        ret = btrfs_reloc_post_snapshot(trans, pending);
        if (ret) {
                btrfs_abort_transaction(trans, ret);
                goto fail;
        }

        /*
         * Do special qgroup accounting for snapshot, as we do some qgroup
         * snapshot hack to do fast snapshot.
         * To co-operate with that hack, we do hack again.
         * Or snapshot will be greatly slowed down by a subtree qgroup rescan
         */
        ret = qgroup_account_snapshot(trans, root, parent_root,
                                      pending->inherit, objectid);
        if (ret < 0)
                goto fail;

        ret = btrfs_insert_dir_item(trans, dentry->d_name.name,
                                    dentry->d_name.len, BTRFS_I(parent_inode),
                                    &key, BTRFS_FT_DIR, index);
        /* We have check then name at the beginning, so it is impossible. */
        BUG_ON(ret == -EEXIST || ret == -EOVERFLOW);
        if (ret) {
                btrfs_abort_transaction(trans, ret);
                goto fail;
        }

        btrfs_i_size_write(BTRFS_I(parent_inode), parent_inode->i_size +
                                         dentry->d_name.len * 2);
        parent_inode->i_mtime = parent_inode->i_ctime =
                current_time(parent_inode);
        ret = btrfs_update_inode_fallback(trans, parent_root, BTRFS_I(parent_inode));
        if (ret) {
                btrfs_abort_transaction(trans, ret);
                goto fail;
        }
        ret = btrfs_uuid_tree_add(trans, new_root_item->uuid,
                                  BTRFS_UUID_KEY_SUBVOL,
                                  objectid);
        if (ret) {
                btrfs_abort_transaction(trans, ret);
                goto fail;
        }
        if (!btrfs_is_empty_uuid(new_root_item->received_uuid)) {
                ret = btrfs_uuid_tree_add(trans, new_root_item->received_uuid,
                                          BTRFS_UUID_KEY_RECEIVED_SUBVOL,
                                          objectid);
                if (ret && ret != -EEXIST) {
                        btrfs_abort_transaction(trans, ret);
                        goto fail;
                }
        }

fail:
        pending->error = ret;
dir_item_existed:
        trans->block_rsv = rsv;
        trans->bytes_reserved = 0;
clear_skip_qgroup:
        btrfs_clear_skip_qgroup(trans);
no_free_objectid:
        kfree(new_root_item);
        pending->root_item = NULL;
        btrfs_free_path(path);
        pending->path = NULL;

        return ret;
}

/*
 * create all the snapshots we've scheduled for creation
 */
static noinline int create_pending_snapshots(struct btrfs_trans_handle *trans)
{
        struct btrfs_pending_snapshot *pending, *next;
        struct list_head *head = &trans->transaction->pending_snapshots;
        int ret = 0;

        list_for_each_entry_safe(pending, next, head, list) {
                list_del(&pending->list);
                ret = create_pending_snapshot(trans, pending);
                if (ret)
                        break;
        }
        return ret;
}

static void update_super_roots(struct btrfs_fs_info *fs_info)
{
        struct btrfs_root_item *root_item;
        struct btrfs_super_block *super;

        super = fs_info->super_copy;

        root_item = &fs_info->chunk_root->root_item;
        super->chunk_root = root_item->bytenr;
        super->chunk_root_generation = root_item->generation;
        super->chunk_root_level = root_item->level;

        root_item = &fs_info->tree_root->root_item;
        super->root = root_item->bytenr;
        super->generation = root_item->generation;
        super->root_level = root_item->level;
        if (btrfs_test_opt(fs_info, SPACE_CACHE))
                super->cache_generation = root_item->generation;
        else if (test_bit(BTRFS_FS_CLEANUP_SPACE_CACHE_V1, &fs_info->flags))
                super->cache_generation = 0;
        if (test_bit(BTRFS_FS_UPDATE_UUID_TREE_GEN, &fs_info->flags))
                super->uuid_tree_generation = root_item->generation;
}

int btrfs_transaction_in_commit(struct btrfs_fs_info *info)
{
        struct btrfs_transaction *trans;
        int ret = 0;

        spin_lock(&info->trans_lock);
        trans = info->running_transaction;
        if (trans)
                ret = (trans->state >= TRANS_STATE_COMMIT_START);
        spin_unlock(&info->trans_lock);
        return ret;
}

int btrfs_transaction_blocked(struct btrfs_fs_info *info)
{
        struct btrfs_transaction *trans;
        int ret = 0;

        spin_lock(&info->trans_lock);
        trans = info->running_transaction;
        if (trans)
                ret = is_transaction_blocked(trans);
        spin_unlock(&info->trans_lock);
        return ret;
}

/*
 * commit transactions asynchronously. once btrfs_commit_transaction_async
 * returns, any subsequent transaction will not be allowed to join.
 */
struct btrfs_async_commit {
        struct btrfs_trans_handle *newtrans;
        struct work_struct work;
};

static void do_async_commit(struct work_struct *work)
{
        struct btrfs_async_commit *ac =
                container_of(work, struct btrfs_async_commit, work);

        /*
         * We've got freeze protection passed with the transaction.
         * Tell lockdep about it.
         */
        if (ac->newtrans->type & __TRANS_FREEZABLE)
                __sb_writers_acquired(ac->newtrans->fs_info->sb, SB_FREEZE_FS);

        current->journal_info = ac->newtrans;

        btrfs_commit_transaction(ac->newtrans);
        kfree(ac);
}

int btrfs_commit_transaction_async(struct btrfs_trans_handle *trans)
{
        struct btrfs_fs_info *fs_info = trans->fs_info;
        struct btrfs_async_commit *ac;
        struct btrfs_transaction *cur_trans;

        ac = kmalloc(sizeof(*ac), GFP_NOFS);
        if (!ac)
                return -ENOMEM;

        INIT_WORK(&ac->work, do_async_commit);
        ac->newtrans = btrfs_join_transaction(trans->root);
        if (IS_ERR(ac->newtrans)) {
                int err = PTR_ERR(ac->newtrans);
                kfree(ac);
                return err;
        }

        /* take transaction reference */
        cur_trans = trans->transaction;
        refcount_inc(&cur_trans->use_count);

        btrfs_end_transaction(trans);

        /*
         * Tell lockdep we've released the freeze rwsem, since the
         * async commit thread will be the one to unlock it.
         */
        if (ac->newtrans->type & __TRANS_FREEZABLE)
                __sb_writers_release(fs_info->sb, SB_FREEZE_FS);

        schedule_work(&ac->work);
        /*
         * Wait for the current transaction commit to start and block
         * subsequent transaction joins
         */
        wait_event(fs_info->transaction_blocked_wait,
                   cur_trans->state >= TRANS_STATE_COMMIT_START ||
                   TRANS_ABORTED(cur_trans));
        if (current->journal_info == trans)
                current->journal_info = NULL;

        btrfs_put_transaction(cur_trans);
        return 0;
}


static void cleanup_transaction(struct btrfs_trans_handle *trans, int err)
{
        struct btrfs_fs_info *fs_info = trans->fs_info;
        struct btrfs_transaction *cur_trans = trans->transaction;

        WARN_ON(refcount_read(&trans->use_count) > 1);

        btrfs_abort_transaction(trans, err);

        spin_lock(&fs_info->trans_lock);

        /*
         * If the transaction is removed from the list, it means this
         * transaction has been committed successfully, so it is impossible
         * to call the cleanup function.
         */
        BUG_ON(list_empty(&cur_trans->list));

        if (cur_trans == fs_info->running_transaction) {
                cur_trans->state = TRANS_STATE_COMMIT_DOING;
                spin_unlock(&fs_info->trans_lock);
                wait_event(cur_trans->writer_wait,
                           atomic_read(&cur_trans->num_writers) == 1);

                spin_lock(&fs_info->trans_lock);
        }

        /*
         * Now that we know no one else is still using the transaction we can
         * remove the transaction from the list of transactions. This avoids
         * the transaction kthread from cleaning up the transaction while some
         * other task is still using it, which could result in a use-after-free
         * on things like log trees, as it forces the transaction kthread to
         * wait for this transaction to be cleaned up by us.
         */
        list_del_init(&cur_trans->list);

        spin_unlock(&fs_info->trans_lock);

        btrfs_cleanup_one_transaction(trans->transaction, fs_info);

        spin_lock(&fs_info->trans_lock);
        if (cur_trans == fs_info->running_transaction)
                fs_info->running_transaction = NULL;
        spin_unlock(&fs_info->trans_lock);

        if (trans->type & __TRANS_FREEZABLE)
                sb_end_intwrite(fs_info->sb);
        btrfs_put_transaction(cur_trans);
        btrfs_put_transaction(cur_trans);

        trace_btrfs_transaction_commit(trans->root);

        if (current->journal_info == trans)
                current->journal_info = NULL;
        btrfs_scrub_cancel(fs_info);

        kmem_cache_free(btrfs_trans_handle_cachep, trans);
}

/*
 * Release reserved delayed ref space of all pending block groups of the
 * transaction and remove them from the list
 */
static void btrfs_cleanup_pending_block_groups(struct btrfs_trans_handle *trans)
{
       struct btrfs_fs_info *fs_info = trans->fs_info;
       struct btrfs_block_group *block_group, *tmp;

       list_for_each_entry_safe(block_group, tmp, &trans->new_bgs, bg_list) {
               btrfs_delayed_refs_rsv_release(fs_info, 1);
               list_del_init(&block_group->bg_list);
       }
}

static inline int btrfs_start_delalloc_flush(struct btrfs_fs_info *fs_info)
{
        /*
         * We use try_to_writeback_inodes_sb() here because if we used
         * btrfs_start_delalloc_roots we would deadlock with fs freeze.
         * Currently are holding the fs freeze lock, if we do an async flush
         * we'll do btrfs_join_transaction() and deadlock because we need to
         * wait for the fs freeze lock.  Using the direct flushing we benefit
         * from already being in a transaction and our join_transaction doesn't
         * have to re-take the fs freeze lock.
         *
         * Note that try_to_writeback_inodes_sb() will only trigger writeback
         * if it can read lock sb->s_umount. It will always be able to lock it,
         * except when the filesystem is being unmounted or being frozen, but in
         * those cases sync_filesystem() is called, which results in calling
         * writeback_inodes_sb() while holding a write lock on sb->s_umount.
         * Note that we don't call writeback_inodes_sb() directly, because it
         * will emit a warning if sb->s_umount is not locked.
         */
        if (btrfs_test_opt(fs_info, FLUSHONCOMMIT))
                try_to_writeback_inodes_sb(fs_info->sb, WB_REASON_SYNC);
        return 0;
}

static inline void btrfs_wait_delalloc_flush(struct btrfs_fs_info *fs_info)
{
        if (btrfs_test_opt(fs_info, FLUSHONCOMMIT))
                btrfs_wait_ordered_roots(fs_info, U64_MAX, 0, (u64)-1);
}

/*
 * Add a pending snapshot associated with the given transaction handle to the
 * respective handle. This must be called after the transaction commit started
 * and while holding fs_info->trans_lock.
 * This serves to guarantee a caller of btrfs_commit_transaction() that it can
 * safely free the pending snapshot pointer in case btrfs_commit_transaction()
 * returns an error.
 */
static void add_pending_snapshot(struct btrfs_trans_handle *trans)
{
        struct btrfs_transaction *cur_trans = trans->transaction;

        if (!trans->pending_snapshot)
                return;

        lockdep_assert_held(&trans->fs_info->trans_lock);
        ASSERT(cur_trans->state >= TRANS_STATE_COMMIT_START);

        list_add(&trans->pending_snapshot->list, &cur_trans->pending_snapshots);
}

int btrfs_commit_transaction(struct btrfs_trans_handle *trans)
{
        struct btrfs_fs_info *fs_info = trans->fs_info;
        struct btrfs_transaction *cur_trans = trans->transaction;
        struct btrfs_transaction *prev_trans = NULL;
        int ret;

        ASSERT(refcount_read(&trans->use_count) == 1);

        /* Stop the commit early if ->aborted is set */
        if (TRANS_ABORTED(cur_trans)) {
                ret = cur_trans->aborted;
                btrfs_end_transaction(trans);
                return ret;
        }

        btrfs_trans_release_metadata(trans);
        trans->block_rsv = NULL;

        /*
         * We only want one transaction commit doing the flushing so we do not
         * waste a bunch of time on lock contention on the extent root node.
         */
        if (!test_and_set_bit(BTRFS_DELAYED_REFS_FLUSHING,
                              &cur_trans->delayed_refs.flags)) {
                /*
                 * Make a pass through all the delayed refs we have so far.
                 * Any running threads may add more while we are here.
                 */
                ret = btrfs_run_delayed_refs(trans, 0);
                if (ret) {
                        btrfs_end_transaction(trans);
                        return ret;
                }
        }

        btrfs_create_pending_block_groups(trans);

        if (!test_bit(BTRFS_TRANS_DIRTY_BG_RUN, &cur_trans->flags)) {
                int run_it = 0;

                /* this mutex is also taken before trying to set
                 * block groups readonly.  We need to make sure
                 * that nobody has set a block group readonly
                 * after a extents from that block group have been
                 * allocated for cache files.  btrfs_set_block_group_ro
                 * will wait for the transaction to commit if it
                 * finds BTRFS_TRANS_DIRTY_BG_RUN set.
                 *
                 * The BTRFS_TRANS_DIRTY_BG_RUN flag is also used to make sure
                 * only one process starts all the block group IO.  It wouldn't
                 * hurt to have more than one go through, but there's no
                 * real advantage to it either.
                 */
                mutex_lock(&fs_info->ro_block_group_mutex);
                if (!test_and_set_bit(BTRFS_TRANS_DIRTY_BG_RUN,
                                      &cur_trans->flags))
                        run_it = 1;
                mutex_unlock(&fs_info->ro_block_group_mutex);

                if (run_it) {
                        ret = btrfs_start_dirty_block_groups(trans);
                        if (ret) {
                                btrfs_end_transaction(trans);
                                return ret;
                        }
                }
        }

        spin_lock(&fs_info->trans_lock);
        if (cur_trans->state >= TRANS_STATE_COMMIT_START) {
                enum btrfs_trans_state want_state = TRANS_STATE_COMPLETED;

                add_pending_snapshot(trans);

                spin_unlock(&fs_info->trans_lock);
                refcount_inc(&cur_trans->use_count);

                if (trans->in_fsync)
                        want_state = TRANS_STATE_SUPER_COMMITTED;
                ret = btrfs_end_transaction(trans);
                wait_for_commit(cur_trans, want_state);

                if (TRANS_ABORTED(cur_trans))
                        ret = cur_trans->aborted;

                btrfs_put_transaction(cur_trans);

                return ret;
        }

        cur_trans->state = TRANS_STATE_COMMIT_START;
        wake_up(&fs_info->transaction_blocked_wait);

        if (cur_trans->list.prev != &fs_info->trans_list) {
                enum btrfs_trans_state want_state = TRANS_STATE_COMPLETED;

                if (trans->in_fsync)
                        want_state = TRANS_STATE_SUPER_COMMITTED;

                prev_trans = list_entry(cur_trans->list.prev,
                                        struct btrfs_transaction, list);
                if (prev_trans->state < want_state) {
                        refcount_inc(&prev_trans->use_count);
                        spin_unlock(&fs_info->trans_lock);

                        wait_for_commit(prev_trans, want_state);

                        ret = READ_ONCE(prev_trans->aborted);

                        btrfs_put_transaction(prev_trans);
                        if (ret)
                                goto cleanup_transaction;
                } else {
                        spin_unlock(&fs_info->trans_lock);
                }
        } else {
                spin_unlock(&fs_info->trans_lock);
                /*
                 * The previous transaction was aborted and was already removed
                 * from the list of transactions at fs_info->trans_list. So we
                 * abort to prevent writing a new superblock that reflects a
                 * corrupt state (pointing to trees with unwritten nodes/leafs).
                 */
                if (test_bit(BTRFS_FS_STATE_TRANS_ABORTED, &fs_info->fs_state)) {
                        ret = -EROFS;
                        goto cleanup_transaction;
                }
        }

        extwriter_counter_dec(cur_trans, trans->type);

        ret = btrfs_start_delalloc_flush(fs_info);
        if (ret)
                goto cleanup_transaction;

        ret = btrfs_run_delayed_items(trans);
        if (ret)
                goto cleanup_transaction;

        wait_event(cur_trans->writer_wait,
                   extwriter_counter_read(cur_trans) == 0);

        /* some pending stuffs might be added after the previous flush. */
        ret = btrfs_run_delayed_items(trans);
        if (ret)
                goto cleanup_transaction;

        btrfs_wait_delalloc_flush(fs_info);

        /*
         * Wait for all ordered extents started by a fast fsync that joined this
         * transaction. Otherwise if this transaction commits before the ordered
         * extents complete we lose logged data after a power failure.
         */
        wait_event(cur_trans->pending_wait,
                   atomic_read(&cur_trans->pending_ordered) == 0);

        btrfs_scrub_pause(fs_info);
        /*
         * Ok now we need to make sure to block out any other joins while we
         * commit the transaction.  We could have started a join before setting
         * COMMIT_DOING so make sure to wait for num_writers to == 1 again.
         */
        spin_lock(&fs_info->trans_lock);
        add_pending_snapshot(trans);
        cur_trans->state = TRANS_STATE_COMMIT_DOING;
        spin_unlock(&fs_info->trans_lock);
        wait_event(cur_trans->writer_wait,
                   atomic_read(&cur_trans->num_writers) == 1);

        if (TRANS_ABORTED(cur_trans)) {
                ret = cur_trans->aborted;
                goto scrub_continue;
        }
        /*
         * the reloc mutex makes sure that we stop
         * the balancing code from coming in and moving
         * extents around in the middle of the commit
         */
        mutex_lock(&fs_info->reloc_mutex);

        /*
         * We needn't worry about the delayed items because we will
         * deal with them in create_pending_snapshot(), which is the
         * core function of the snapshot creation.
         */
        ret = create_pending_snapshots(trans);
        if (ret)
                goto unlock_reloc;

        /*
         * We insert the dir indexes of the snapshots and update the inode
         * of the snapshots' parents after the snapshot creation, so there
         * are some delayed items which are not dealt with. Now deal with
         * them.
         *
         * We needn't worry that this operation will corrupt the snapshots,
         * because all the tree which are snapshoted will be forced to COW
         * the nodes and leaves.
         */
        ret = btrfs_run_delayed_items(trans);
        if (ret)
                goto unlock_reloc;

        ret = btrfs_run_delayed_refs(trans, (unsigned long)-1);
        if (ret)
                goto unlock_reloc;

        /*
         * make sure none of the code above managed to slip in a
         * delayed item
         */
        btrfs_assert_delayed_root_empty(fs_info);

        WARN_ON(cur_trans != trans->transaction);

        /* btrfs_commit_tree_roots is responsible for getting the
         * various roots consistent with each other.  Every pointer
         * in the tree of tree roots has to point to the most up to date
         * root for every subvolume and other tree.  So, we have to keep
         * the tree logging code from jumping in and changing any
         * of the trees.
         *
         * At this point in the commit, there can't be any tree-log
         * writers, but a little lower down we drop the trans mutex
         * and let new people in.  By holding the tree_log_mutex
         * from now until after the super is written, we avoid races
         * with the tree-log code.
         */
        mutex_lock(&fs_info->tree_log_mutex);

        ret = commit_fs_roots(trans);
        if (ret)
                goto unlock_tree_log;

        /*
         * Since the transaction is done, we can apply the pending changes
         * before the next transaction.
         */
        btrfs_apply_pending_changes(fs_info);

        /* commit_fs_roots gets rid of all the tree log roots, it is now
         * safe to free the root of tree log roots
         */
        btrfs_free_log_root_tree(trans, fs_info);

        /*
         * Since fs roots are all committed, we can get a quite accurate
         * new_roots. So let's do quota accounting.
         */
        ret = btrfs_qgroup_account_extents(trans);
        if (ret < 0)
                goto unlock_tree_log;

        ret = commit_cowonly_roots(trans);
        if (ret)
                goto unlock_tree_log;

        /*
         * The tasks which save the space cache and inode cache may also
         * update ->aborted, check it.
         */
        if (TRANS_ABORTED(cur_trans)) {
                ret = cur_trans->aborted;
                goto unlock_tree_log;
        }

        cur_trans = fs_info->running_transaction;

        btrfs_set_root_node(&fs_info->tree_root->root_item,
                            fs_info->tree_root->node);
        list_add_tail(&fs_info->tree_root->dirty_list,
                      &cur_trans->switch_commits);

        btrfs_set_root_node(&fs_info->chunk_root->root_item,
                            fs_info->chunk_root->node);
        list_add_tail(&fs_info->chunk_root->dirty_list,
                      &cur_trans->switch_commits);

        switch_commit_roots(trans);

        ASSERT(list_empty(&cur_trans->dirty_bgs));
        ASSERT(list_empty(&cur_trans->io_bgs));
        update_super_roots(fs_info);

        btrfs_set_super_log_root(fs_info->super_copy, 0);
        btrfs_set_super_log_root_level(fs_info->super_copy, 0);
        memcpy(fs_info->super_for_commit, fs_info->super_copy,
               sizeof(*fs_info->super_copy));

        btrfs_commit_device_sizes(cur_trans);

        clear_bit(BTRFS_FS_LOG1_ERR, &fs_info->flags);
        clear_bit(BTRFS_FS_LOG2_ERR, &fs_info->flags);

        btrfs_trans_release_chunk_metadata(trans);

        spin_lock(&fs_info->trans_lock);
        cur_trans->state = TRANS_STATE_UNBLOCKED;
        fs_info->running_transaction = NULL;
        spin_unlock(&fs_info->trans_lock);
        mutex_unlock(&fs_info->reloc_mutex);

        wake_up(&fs_info->transaction_wait);

        ret = btrfs_write_and_wait_transaction(trans);
        if (ret) {
                btrfs_handle_fs_error(fs_info, ret,
                                      "Error while writing out transaction");
                /*
                 * reloc_mutex has been unlocked, tree_log_mutex is still held
                 * but we can't jump to unlock_tree_log causing double unlock
                 */
                mutex_unlock(&fs_info->tree_log_mutex);
                goto scrub_continue;
        }

        /*
         * At this point, we should have written all the tree blocks allocated
         * in this transaction. So it's now safe to free the redirtyied extent
         * buffers.
         */
        btrfs_free_redirty_list(cur_trans);

        ret = write_all_supers(fs_info, 0);
        /*
         * the super is written, we can safely allow the tree-loggers
         * to go about their business
         */
        mutex_unlock(&fs_info->tree_log_mutex);
        if (ret)
                goto scrub_continue;

        /*
         * We needn't acquire the lock here because there is no other task
         * which can change it.
         */
        cur_trans->state = TRANS_STATE_SUPER_COMMITTED;
        wake_up(&cur_trans->commit_wait);

        btrfs_finish_extent_commit(trans);

        if (test_bit(BTRFS_TRANS_HAVE_FREE_BGS, &cur_trans->flags))
                btrfs_clear_space_info_full(fs_info);

        fs_info->last_trans_committed = cur_trans->transid;
        /*
         * We needn't acquire the lock here because there is no other task
         * which can change it.
         */
        cur_trans->state = TRANS_STATE_COMPLETED;
        wake_up(&cur_trans->commit_wait);

        spin_lock(&fs_info->trans_lock);
        list_del_init(&cur_trans->list);
        spin_unlock(&fs_info->trans_lock);

        btrfs_put_transaction(cur_trans);
        btrfs_put_transaction(cur_trans);

        if (trans->type & __TRANS_FREEZABLE)
                sb_end_intwrite(fs_info->sb);

        trace_btrfs_transaction_commit(trans->root);

        btrfs_scrub_continue(fs_info);

        if (current->journal_info == trans)
                current->journal_info = NULL;

        kmem_cache_free(btrfs_trans_handle_cachep, trans);

        return ret;

unlock_tree_log:
        mutex_unlock(&fs_info->tree_log_mutex);
unlock_reloc:
        mutex_unlock(&fs_info->reloc_mutex);
scrub_continue:
        btrfs_scrub_continue(fs_info);
cleanup_transaction:
        btrfs_trans_release_metadata(trans);
        btrfs_cleanup_pending_block_groups(trans);
        btrfs_trans_release_chunk_metadata(trans);
        trans->block_rsv = NULL;
        btrfs_warn(fs_info, "Skipping commit of aborted transaction.");
        if (current->journal_info == trans)
                current->journal_info = NULL;
        cleanup_transaction(trans, ret);

        return ret;
}

/*
 * return < 0 if error
 * 0 if there are no more dead_roots at the time of call
 * 1 there are more to be processed, call me again
 *
 * The return value indicates there are certainly more snapshots to delete, but
 * if there comes a new one during processing, it may return 0. We don't mind,
 * because btrfs_commit_super will poke cleaner thread and it will process it a
 * few seconds later.
 */
int btrfs_clean_one_deleted_snapshot(struct btrfs_root *root)
{
        int ret;
        struct btrfs_fs_info *fs_info = root->fs_info;

        spin_lock(&fs_info->trans_lock);
        if (list_empty(&fs_info->dead_roots)) {
                spin_unlock(&fs_info->trans_lock);
                return 0;
        }
        root = list_first_entry(&fs_info->dead_roots,
                        struct btrfs_root, root_list);
        list_del_init(&root->root_list);
        spin_unlock(&fs_info->trans_lock);

        btrfs_debug(fs_info, "cleaner removing %llu", root->root_key.objectid);

        btrfs_kill_all_delayed_nodes(root);

        if (btrfs_header_backref_rev(root->node) <
                        BTRFS_MIXED_BACKREF_REV)
                ret = btrfs_drop_snapshot(root, 0, 0);
        else
                ret = btrfs_drop_snapshot(root, 1, 0);

        btrfs_put_root(root);
        return (ret < 0) ? 0 : 1;
}

void btrfs_apply_pending_changes(struct btrfs_fs_info *fs_info)
{
        unsigned long prev;
        unsigned long bit;

        prev = xchg(&fs_info->pending_changes, 0);
        if (!prev)
                return;

        bit = 1 << BTRFS_PENDING_COMMIT;
        if (prev & bit)
                btrfs_debug(fs_info, "pending commit done");
        prev &= ~bit;

        if (prev)
                btrfs_warn(fs_info,
                        "unknown pending changes left 0x%lx, ignoring", prev);
}