From: Filipe Manana fdmanana@suse.com
commit ebb929060aeb162417b4c1307e63daee47b208d9 upstream.
When we are doing a full fsync (bit BTRFS_INODE_NEEDS_FULL_SYNC set) of a file that has holes and has file extent items spanning two or more leafs, we can end up falling to back to a full transaction commit due to a logic bug that leads to failure to insert a duplicate file extent item that is meant to represent a hole between the last file extent item of a leaf and the first file extent item in the next leaf. The failure (EEXIST error) leads to a transaction commit (as most errors when logging an inode do).
For example, we have the two following leafs:
Leaf N:
----------------------------------------------- | ..., ..., ..., (257, FILE_EXTENT_ITEM, 64K) | ----------------------------------------------- The file extent item at the end of leaf N has a length of 4Kb, representing the file range from 64K to 68K - 1.
Leaf N + 1:
----------------------------------------------- | (257, FILE_EXTENT_ITEM, 72K), ..., ..., ... | ----------------------------------------------- The file extent item at the first slot of leaf N + 1 has a length of 4Kb too, representing the file range from 72K to 76K - 1.
During the full fsync path, when we are at tree-log.c:copy_items() with leaf N as a parameter, after processing the last file extent item, that represents the extent at offset 64K, we take a look at the first file extent item at the next leaf (leaf N + 1), and notice there's a 4K hole between the two extents, and therefore we insert a file extent item representing that hole, starting at file offset 68K and ending at offset 72K - 1. However we don't update the value of *last_extent, which is used to represent the end offset (plus 1, non-inclusive end) of the last file extent item inserted in the log, so it stays with a value of 68K and not with a value of 72K.
Then, when copy_items() is called for leaf N + 1, because the value of *last_extent is smaller then the offset of the first extent item in the leaf (68K < 72K), we look at the last file extent item in the previous leaf (leaf N) and see it there's a 4K gap between it and our first file extent item (again, 68K < 72K), so we decide to insert a file extent item representing the hole, starting at file offset 68K and ending at offset 72K - 1, this insertion will fail with -EEXIST being returned from btrfs_insert_file_extent() because we already inserted a file extent item representing a hole for this offset (68K) in the previous call to copy_items(), when processing leaf N.
The -EEXIST error gets propagated to the fsync callback, btrfs_sync_file(), which falls back to a full transaction commit.
Fix this by adjusting *last_extent after inserting a hole when we had to look at the next leaf.
Fixes: 4ee3fad34a9c ("Btrfs: fix fsync after hole punching when using no-holes feature") Cc: stable@vger.kernel.org # 4.14+ 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: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/btrfs/tree-log.c | 1 + 1 file changed, 1 insertion(+)
--- a/fs/btrfs/tree-log.c +++ b/fs/btrfs/tree-log.c @@ -4121,6 +4121,7 @@ fill_holes: *last_extent, 0, 0, len, 0, len, 0, 0, 0); + *last_extent += len; } } }