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>

diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
index 2a48ea3f1b30d4adfc6581dff3d1cfe1a088b86a..b6b5acc92ca2da725de6a7b0f867c7c83b77d464 100644 (file)
--- a/drivers/md/dm-bufio.c
+++ b/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);