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    Subject[PATCH 5.7 028/204] btrfs: fix race between page release and a fast fsync
    Date
    From: Filipe Manana <fdmanana@suse.com>

    commit 3d6448e631591756da36efb3ea6355ff6f383c3a upstream.

    When releasing an extent map, done through the page release callback, we
    can race with an ongoing fast fsync and cause the fsync to miss a new
    extent and not log it. The steps for this to happen are the following:

    1) A page is dirtied for some inode I;

    2) Writeback for that page is triggered by a path other than fsync, for
    example by the system due to memory pressure;

    3) When the ordered extent for the extent (a single 4K page) finishes,
    we unpin the corresponding extent map and set its generation to N,
    the current transaction's generation;

    4) The btrfs_releasepage() callback is invoked by the system due to
    memory pressure for that no longer dirty page of inode I;

    5) At the same time, some task calls fsync on inode I, joins transaction
    N, and at btrfs_log_inode() it sees that the inode does not have the
    full sync flag set, so we proceed with a fast fsync. But before we get
    into btrfs_log_changed_extents() and lock the inode's extent map tree:

    6) Through btrfs_releasepage() we end up at try_release_extent_mapping()
    and we remove the extent map for the new 4Kb extent, because it is
    neither pinned anymore nor locked. By calling remove_extent_mapping(),
    we remove the extent map from the list of modified extents, since the
    extent map does not have the logging flag set. We unlock the inode's
    extent map tree;

    7) The task doing the fast fsync now enters btrfs_log_changed_extents(),
    locks the inode's extent map tree and iterates its list of modified
    extents, which no longer has the 4Kb extent in it, so it does not log
    the extent;

    8) The fsync finishes;

    9) Before transaction N is committed, a power failure happens. After
    replaying the log, the 4K extent of inode I will be missing, since
    it was not logged due to the race with try_release_extent_mapping().

    So fix this by teaching try_release_extent_mapping() to not remove an
    extent map if it's still in the list of modified extents.

    Fixes: ff44c6e36dc9dc ("Btrfs: do not hold the write_lock on the extent tree while logging")
    CC: stable@vger.kernel.org # 5.4+
    Signed-off-by: Filipe Manana <fdmanana@suse.com>
    Signed-off-by: David Sterba <dsterba@suse.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    fs/btrfs/extent_io.c | 16 +++++++++++++---
    1 file changed, 13 insertions(+), 3 deletions(-)

    --- a/fs/btrfs/extent_io.c
    +++ b/fs/btrfs/extent_io.c
    @@ -4504,15 +4504,25 @@ int try_release_extent_mapping(struct pa
    free_extent_map(em);
    break;
    }
    - if (!test_range_bit(tree, em->start,
    - extent_map_end(em) - 1,
    - EXTENT_LOCKED, 0, NULL)) {
    + if (test_range_bit(tree, em->start,
    + extent_map_end(em) - 1,
    + EXTENT_LOCKED, 0, NULL))
    + goto next;
    + /*
    + * If it's not in the list of modified extents, used
    + * by a fast fsync, we can remove it. If it's being
    + * logged we can safely remove it since fsync took an
    + * extra reference on the em.
    + */
    + if (list_empty(&em->list) ||
    + test_bit(EXTENT_FLAG_LOGGING, &em->flags)) {
    set_bit(BTRFS_INODE_NEEDS_FULL_SYNC,
    &btrfs_inode->runtime_flags);
    remove_extent_mapping(map, em);
    /* once for the rb tree */
    free_extent_map(em);
    }
    +next:
    start = extent_map_end(em);
    write_unlock(&map->lock);


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