dm bufio: avoid sleeping while holding the dm_bufio lock

We've seen in-field reports showing _lots_ (18 in one case, 41 in
another) of tasks all sitting there blocked on:

  mutex_lock+0x4c/0x68
  dm_bufio_shrink_count+0x38/0x78
  shrink_slab.part.54.constprop.65+0x100/0x464
  shrink_zone+0xa8/0x198

In the two cases analyzed, we see one task that looks like this:

  Workqueue: kverityd verity_prefetch_io

  __switch_to+0x9c/0xa8
  __schedule+0x440/0x6d8
  schedule+0x94/0xb4
  schedule_timeout+0x204/0x27c
  schedule_timeout_uninterruptible+0x44/0x50
  wait_iff_congested+0x9c/0x1f0
  shrink_inactive_list+0x3a0/0x4cc
  shrink_lruvec+0x418/0x5cc
  shrink_zone+0x88/0x198
  try_to_free_pages+0x51c/0x588
  __alloc_pages_nodemask+0x648/0xa88
  __get_free_pages+0x34/0x7c
  alloc_buffer+0xa4/0x144
  __bufio_new+0x84/0x278
  dm_bufio_prefetch+0x9c/0x154
  verity_prefetch_io+0xe8/0x10c
  process_one_work+0x240/0x424
  worker_thread+0x2fc/0x424
  kthread+0x10c/0x114

...and that looks to be the one holding the mutex.

The problem has been reproduced on fairly easily:
0. Be running Chrome OS w/ verity enabled on the root filesystem
1. Pick test patch: http://crosreview.com/412360
2. Install launchBalloons.sh and balloon.arm from
     http://crbug.com/468342
   ...that's just a memory stress test app.
3. On a 4GB rk3399 machine, run
     nice ./launchBalloons.sh 4 900 100000
   ...that tries to eat 4 * 900 MB of memory and keep accessing.
4. Login to the Chrome web browser and restore many tabs

With that, I've seen printouts like:
  DOUG: long bufio 90758 ms
...and stack trace always show's we're in dm_bufio_prefetch().

The problem is that we try to allocate memory with GFP_NOIO while
we're holding the dm_bufio lock.  Instead we should be using
GFP_NOWAIT.  Using GFP_NOIO can cause us to sleep while holding the
lock and that causes the above problems.

The current behavior explained by David Rientjes:

  It will still try reclaim initially because __GFP_WAIT (or
  __GFP_KSWAPD_RECLAIM) is set by GFP_NOIO.  This is the cause of
  contention on dm_bufio_lock() that the thread holds.  You want to
  pass GFP_NOWAIT instead of GFP_NOIO to alloc_buffer() when holding a
  mutex that can be contended by a concurrent slab shrinker (if
  count_objects didn't use a trylock, this pattern would trivially
  deadlock).

This change significantly increases responsiveness of the system while
in this state.  It makes a real difference because it unblocks kswapd.
In the bug report analyzed, kswapd was hung:

   kswapd0         D ffffffc000204fd8     0    72      2 0x00000000
   Call trace:
   [<ffffffc000204fd8>] __switch_to+0x9c/0xa8
   [<ffffffc00090b794>] __schedule+0x440/0x6d8
   [<ffffffc00090bac0>] schedule+0x94/0xb4
   [<ffffffc00090be44>] schedule_preempt_disabled+0x28/0x44
   [<ffffffc00090d900>] __mutex_lock_slowpath+0x120/0x1ac
   [<ffffffc00090d9d8>] mutex_lock+0x4c/0x68
   [<ffffffc000708e7c>] dm_bufio_shrink_count+0x38/0x78
   [<ffffffc00030b268>] shrink_slab.part.54.constprop.65+0x100/0x464
   [<ffffffc00030dbd8>] shrink_zone+0xa8/0x198
   [<ffffffc00030e578>] balance_pgdat+0x328/0x508
   [<ffffffc00030eb7c>] kswapd+0x424/0x51c
   [<ffffffc00023f06c>] kthread+0x10c/0x114
   [<ffffffc000203dd0>] ret_from_fork+0x10/0x40

By unblocking kswapd memory pressure should be reduced.
Suggested-by: David Rientjes <rientjes@google.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-bufio.c

@@ -827,7 +827,8 @@ static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client
 	 * dm-bufio is resistant to allocation failures (it just keeps
 	 * one buffer reserved in cases all the allocations fail).
 	 * So set flags to not try too hard:
-	 *	GFP_NOIO: don't recurse into the I/O layer
+	 *	GFP_NOWAIT: don't wait; if we need to sleep we'll release our
+	 *		    mutex and wait ourselves.
 	 *	__GFP_NORETRY: don't retry and rather return failure
 	 *	__GFP_NOMEMALLOC: don't use emergency reserves
 	 *	__GFP_NOWARN: don't print a warning in case of failure
@@ -837,7 +838,7 @@ static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client
 	 */
 	while (1) {
 		if (dm_bufio_cache_size_latch != 1) {
-			b = alloc_buffer(c, GFP_NOIO | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
+			b = alloc_buffer(c, GFP_NOWAIT | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
 			if (b)
 				return b;
 		}