[SRU] [B/C] [PATCH 0/1] btrfs: raid56: fix page unmapping bug

[Andrea Righi]

[Impact]

Parity page in btrfs raid56 is incorrectly unmapped, allowing to
easily trigger a reference counter bug on i386 causing a kernel panic.

The fix unmaps the right rbio pages and adds the proper kunmap() call
for the parity page.

[Test Case]

create a raid5 btrfs filesystem:
  # mkfs.btrfs -m raid5 -d raid5 /dev/sdb /dev/sdc /dev/sdd /dev/sde

mount it:
  # mount /dev/sdb /mnt

run btrfs scrub in a loop:
  # while :; do btrfs scrub start -BR /mnt; done

[Fix]

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=3897b6f0a859288c22fb793fad11ec2327e60fcd

kunmap(p_page) was completely left out, so we never did an unmap for the
p_page and the loop unmapping the rbio page was iterating over the wrong
number of stripes: unmapping should be done with nr_data instead of
rbio->real_stripes.

[Regression Potential]

This is an upstream fix, tested on the affected platform. The bug is
affecting only btrfs raid5/6 users on architectures where kunamp() is
not a no-op (like i386). It is also a very small patch, so backport
changes are minimal.

Andrea Righi (1):
  btrfs: raid56: properly unmap parity page in finish_parity_scrub()

 raid56.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)