In the patch 78c52d9e ("btrfs: check for refs on snapshot delete
resume") I added some code to handle file systems that had been
corrupted by a bug that incorrectly skipped updating the drop progress
key while dropping a snapshot.  This code would check to see if we had
already deleted our reference for a child block, and skip the deletion
if we had already.

Unfortunately there is a bug, as the check would only check the on-disk
references.  I made an incorrect assumption that blocks in an already
deleted snapshot that was having the deletion resume on mount wouldn't
be modified.

If we have 2 pending deleted snapshots that share blocks, we can easily
modify the rules for a block.  Take the following example

subvolume a exists, and subvolume b is a snapshot of subvolume a.  They
share references to block 1.  Block 1 will have 2 full references, one
for subvolume a and one for subvolume b, and it belongs to subvolume a
(btrfs_header_owner(block 1) == subvolume a).

When deleting subvolume a, we will drop our full reference for block 1,
and because we are the owner we will drop our full reference for all of
block 1's children, convert block 1 to FULL BACKREF, and add a shared
reference to all of block 1's children.

Then we will start the snapshot deletion of subvolume b.  We look up the
extent info for block 1, which checks delayed refs and tells us that
FULL BACKREF is set, so sets parent to the bytenr of block 1.  However
because this is a resumed snapshot deletion, we call into
check_ref_exists().  Because check_ref_exists() only looks at the disk,
it doesn't find the shared backref for the child of block 1, and thus
returns 0 and we skip deleting the reference for the child of block 1
and continue.  This orphans the child of block 1.

The fix is to lookup the delayed refs, similar to what we do in
btrfs_lookup_extent_info().  However we only care about whether the
reference exists or not.  If we fail to find our reference on disk, go
look up the bytenr in the delayed refs, and if it exists look for an
existing ref in the delayed ref head.  If that exists then we know we
can delete the reference safely and carry on.  If it doesn't exist we
know we have to skip over this block.

This bug has existed since I introduced this fix, however requires
having multiple deleted snapshots pending when we unmount.  We noticed
this in production because our shutdown path stops the container on the
system, which deletes a bunch of subvolumes, and then reboots the box.
This gives us plenty of opportunities to hit this issue.  Looking at the
history we've seen this occasionally in production, but we had a big
spike recently thanks to faster machines getting jobs with multiple
subvolumes in the job.

Chris Mason wrote a reproducer which does the following

mount /dev/nvme4n1 /btrfs
btrfs subvol create /btrfs/s1
simoop -E -f 4k -n 200000 -z /btrfs/s1
while(true) ; do
	btrfs subvol snap /btrfs/s1 /btrfs/s2
	simoop -f 4k -n 200000 -r 10 -z /btrfs/s2
	btrfs subvol snap /btrfs/s2 /btrfs/s3
	btrfs balance start -dusage=80 /btrfs
	btrfs subvol del /btrfs/s2 /btrfs/s3
	umount /btrfs
	btrfsck /dev/nvme4n1 || exit 1
	mount /dev/nvme4n1 /btrfs
done

On the second loop this would fail consistently, with my patch it has
been running for hours and hasn't failed.

I also used dm-log-writes to capture the state of the failure so I could
debug the problem.  Using the existing failure case to test my patch
validated that it fixes the problem.

Fixes: 78c52d9e ("btrfs: check for refs on snapshot delete resume")
CC: stable@vger.kernel.org # 5.4+
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>