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    Subject[PATCH 5.13 301/380] btrfs: remove racy and unnecessary inode transaction update when using no-holes
    Date
    From: Filipe Manana <fdmanana@suse.com>

    [ Upstream commit cceaa89f02f15f232391ae4be214137b0a0285c0 ]

    When using the NO_HOLES feature and expanding the size of an inode, we
    update the inode's last_trans, last_sub_trans and last_log_commit fields
    at maybe_insert_hole() so that a fsync does know that the inode needs to
    be logged (by making sure that btrfs_inode_in_log() returns false). This
    happens for expanding truncate operations, buffered writes, direct IO
    writes and when cloning extents to an offset greater than the inode's
    i_size.

    However the way we do it is racy, because in between setting the inode's
    last_sub_trans and last_log_commit fields, the log transaction ID that was
    assigned to last_sub_trans might be committed before we read the root's
    last_log_commit and assign that value to last_log_commit. If that happens
    it would make a future call to btrfs_inode_in_log() return true. This is
    a race that should be extremely unlikely to be hit in practice, and it is
    the same that was described by commit bc0939fcfab0d7 ("btrfs: fix race
    between marking inode needs to be logged and log syncing").

    The fix would simply be to set last_log_commit to the value we assigned
    to last_sub_trans minus 1, like it was done in that commit. However
    updating these two fields plus the last_trans field is pointless here
    because all the callers of btrfs_cont_expand() (which is the only
    caller of maybe_insert_hole()) always call btrfs_set_inode_last_trans()
    or btrfs_update_inode() after calling btrfs_cont_expand(). Calling either
    btrfs_set_inode_last_trans() or btrfs_update_inode() guarantees that the
    next fsync will log the inode, as it makes btrfs_inode_in_log() return
    false.

    So just remove the code that explicitly sets the inode's last_trans,
    last_sub_trans and last_log_commit fields.

    Reviewed-by: Josef Bacik <josef@toxicpanda.com>
    Signed-off-by: Filipe Manana <fdmanana@suse.com>
    Signed-off-by: David Sterba <dsterba@suse.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    fs/btrfs/inode.c | 12 +++++-------
    1 file changed, 5 insertions(+), 7 deletions(-)

    diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
    index c3810c6430d9..044300db5e22 100644
    --- a/fs/btrfs/inode.c
    +++ b/fs/btrfs/inode.c
    @@ -5064,15 +5064,13 @@ static int maybe_insert_hole(struct btrfs_root *root, struct btrfs_inode *inode,
    int ret;

    /*
    - * Still need to make sure the inode looks like it's been updated so
    - * that any holes get logged if we fsync.
    + * If NO_HOLES is enabled, we don't need to do anything.
    + * Later, up in the call chain, either btrfs_set_inode_last_sub_trans()
    + * or btrfs_update_inode() will be called, which guarantee that the next
    + * fsync will know this inode was changed and needs to be logged.
    */
    - if (btrfs_fs_incompat(fs_info, NO_HOLES)) {
    - inode->last_trans = fs_info->generation;
    - inode->last_sub_trans = root->log_transid;
    - inode->last_log_commit = root->last_log_commit;
    + if (btrfs_fs_incompat(fs_info, NO_HOLES))
    return 0;
    - }

    /*
    * 1 - for the one we're dropping
    --
    2.30.2


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