3.16.61-rc1 review patch. If anyone has any objections, please let me know.
------------------
From: Douglas Anderson dianders@chromium.org
commit 9ea61cac0b1ad0c09022f39fd97e9b99a2cfc2dc upstream.
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 Cc: Mikulas Patocka mpatocka@redhat.com Signed-off-by: Ben Hutchings ben@decadent.org.uk --- drivers/md/dm-bufio.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-)
--- a/drivers/md/dm-bufio.c +++ b/drivers/md/dm-bufio.c @@ -778,7 +778,8 @@ static struct dm_buffer *__alloc_buffer_ * 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 @@ -788,7 +789,7 @@ static struct dm_buffer *__alloc_buffer_ */ 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; }