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);