Btrfs: fix missing inode i_size update after zero range operation

From: Filipe Manana <[email protected]>

For a fallocate's zero range operation that targets a range with an end that is not aligned to the sector size, we can end up not updating the inode's i_size. This happens when the last page of the range maps to an unwritten (prealloc) extent and before that last page we have either a hole or a written extent. This is because in this scenario we relied on a call to btrfs_prealloc_file_range() to update the inode's i_size, however it can only update the i_size to the "down aligned" end of the range.

Example:

 $ mkfs.btrfs -f /dev/sdc
 $ mount /dev/sdc /mnt
 $ xfs_io -f -c "pwrite -S 0xff 0 428K" /mnt/foobar
 $ xfs_io -c "falloc -k 428K 4K" /mnt/foobar
 $ xfs_io -c "fzero 0 430K" /mnt/foobar
 $ du --bytes /mnt/foobar
 438272	/mnt/foobar

The inode's i_size was left as 428Kb (438272 bytes) when it should have been updated to 430Kb (440320 bytes).
Fix this by always updating the inode's i_size explicitly after zeroing the range.

---
 fs/btrfs/file.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

index dc95d9590d2d..9ad0465d2e8e 100644
@@ -3026,9 +3026,12 @@ static int btrfs_zero_range(struct inode *inode,
 		unlock_extent_cached(&BTRFS_I(inode)->io_tree, lockstart,
 				     lockend, &cached_state, GFP_KERNEL);
 		/* btrfs_prealloc_file_range releases reserved space on error */
-		if (ret)
+		if (ret) {
 			space_reserved = false;
+			goto out;
+		}
 	}
+	ret = btrfs_fallocate_update_isize(inode, offset + len, mode);
  out:
 	if (ret && space_reserved)
 		btrfs_free_reserved_data_space(inode, data_reserved,

2.11.0

Added to 4.16 queue, thanks.