dm bufio: fix deadlock with loop device

When thin-volume is built on loop device, if available memory is low,
the following deadlock can be triggered:

One process P1 allocates memory with GFP_FS flag, direct alloc fails,
memory reclaim invokes memory shrinker in dm_bufio, dm_bufio_shrink_scan()
runs, mutex dm_bufio_client->lock is acquired, then P1 waits for dm_buffer
IO to complete in __try_evict_buffer().

But this IO may never complete if issued to an underlying loop device
that forwards it using direct-IO, which allocates memory using
GFP_KERNEL (see: do_blockdev_direct_IO()).  If allocation fails, memory
reclaim will invoke memory shrinker in dm_bufio, dm_bufio_shrink_scan()
will be invoked, and since the mutex is already held by P1 the loop
thread will hang, and IO will never complete.  Resulting in ABBA
deadlock.

Cc: stable@vger.kernel.org
Signed-off-by: Junxiao Bi <junxiao.bi@oracle.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-bufio.c

@@ -1599,9 +1599,7 @@ dm_bufio_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 	unsigned long freed;
 
 	c = container_of(shrink, struct dm_bufio_client, shrinker);
-	if (sc->gfp_mask & __GFP_FS)
-		dm_bufio_lock(c);
-	else if (!dm_bufio_trylock(c))
+	if (!dm_bufio_trylock(c))
 		return SHRINK_STOP;
 
 	freed  = __scan(c, sc->nr_to_scan, sc->gfp_mask);