Patch "btrfs: add missing initialization in btrfs_check_shared" has been added to the 4.14-stable tree

This is a note to let you know that I've just added the patch titled

btrfs: add missing initialization in btrfs_check_shared

to the 4.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
btrfs-add-missing-initialization-in-btrfs_check_shared.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Edmund Nadolski <[email protected]>
Date: Wed, 14 Mar 2018 09:03:11 -0600
Subject: btrfs: add missing initialization in btrfs_check_shared

From: Edmund Nadolski <[email protected]>

commit 18bf591ba9753e3e5ba91f38f756a800693408f4 upstream.

This patch addresses an issue that causes fiemap to falsely report a shared extent. The test case is as follows:

xfs_io -f -d -c "pwrite -b 16k 0 64k" -c "fiemap -v" /media/scratch/file5 sync
xfs_io -c "fiemap -v" /media/scratch/file5

which gives the resulting output:

wrote 65536/65536 bytes at offset 0
64 KiB, 4 ops; 0.0000 sec (121.359 MiB/sec and 7766.9903 ops/sec) /media/scratch/file5:
EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS
0: [0..127]: 24576..24703 128 0x2001
/media/scratch/file5:
EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS
0: [0..127]: 24576..24703 128 0x1

This is because btrfs_check_shared calls find_parent_nodes repeatedly in a loop, passing a share_check struct to report the count of shared extent. But btrfs_check_shared does not re-initialize the count value to zero for subsequent calls from the loop, resulting in a false share count value. This is a regressive behavior from 4.13.

With proper re-initialization the test result is as follows:

wrote 65536/65536 bytes at offset 0
64 KiB, 4 ops; 0.0000 sec (110.035 MiB/sec and 7042.2535 ops/sec) /media/scratch/file5:
EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS
0: [0..127]: 24576..24703 128 0x1
/media/scratch/file5:
EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS
0: [0..127]: 24576..24703 128 0x1

which corrects the regression.

---
 fs/btrfs/backref.c |    1 +
 1 file changed, 1 insertion(+)

@@ -1496,6 +1496,7 @@ int btrfs_check_shared(struct btrfs_root
 		if (!node)
 			break;
 		bytenr = node->val;
+		shared.share_count = 0;
 		cond_resched();
 	}
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/btrfs-add-missing-initialization-in-btrfs_check_shared.patch