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 4.15-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...

The filename of the patch is: btrfs-add-missing-initialization-in-btrfs_check_shared.patch and it can be found in the queue-4.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

From 18bf591ba9753e3e5ba91f38f756a800693408f4 Mon Sep 17 00:00:00 2001

From: Edmund Nadolski enadolski@suse.com Date: Wed, 14 Mar 2018 09:03:11 -0600 Subject: btrfs: add missing initialization in btrfs_check_shared

From: Edmund Nadolski enadolski@suse.com

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.

Fixes: 3ec4d3238ab ("btrfs: allow backref search checks for shared extents") Signed-off-by: Edmund Nadolski enadolski@suse.com [ add text from cover letter to changelog ] Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org

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

--- a/fs/btrfs/backref.c +++ b/fs/btrfs/backref.c @@ -1509,6 +1509,7 @@ 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 enadolski@suse.com are

queue-4.15/btrfs-add-missing-initialization-in-btrfs_check_shared.patch