BACKPORT: dm bufio: don't take the lock in dm_bufio_shrink_count

2017-12-10T13:25:12+00:00

dm_bufio_shrink_count() is called from do_shrink_slab to find out how many freeable objects are there. The reported value doesn't have to be precise, so we don't need to take the dm-bufio lock. Suggested-by: David Rientjes Signed-off-by: Mikulas Patocka Signed-off-by: Mike Snitzer Bug: 64122284 Change-Id: Id2c3446e03e865f424be8666b1ee0822b9e33a63 (cherry picked from commit d12067f428c037b4575aaeb2be00847fc214c24a) Signed-off-by: Sami Tolvanen Signed-off-by: Francisco Franco

BACKPORT: dm bufio: avoid sleeping while holding the dm_bufio lock

2017-12-10T13:24:54+00:00

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: [] __switch_to+0x9c/0xa8 [] __schedule+0x440/0x6d8 [] schedule+0x94/0xb4 [] schedule_preempt_disabled+0x28/0x44 [] __mutex_lock_slowpath+0x120/0x1ac [] mutex_lock+0x4c/0x68 [] dm_bufio_shrink_count+0x38/0x78 [] shrink_slab.part.54.constprop.65+0x100/0x464 [] shrink_zone+0xa8/0x198 [] balance_pgdat+0x328/0x508 [] kswapd+0x424/0x51c [] kthread+0x10c/0x114 [] ret_from_fork+0x10/0x40 By unblocking kswapd memory pressure should be reduced. Suggested-by: David Rientjes Reviewed-by: Guenter Roeck Signed-off-by: Douglas Anderson Signed-off-by: Mike Snitzer Bug: 64122284 Change-Id: I1ce9367c921d7ab07ca9e3d403c95cd0d333915c (cherry picked from commit 9ea61cac0b1ad0c09022f39fd97e9b99a2cfc2dc) Signed-off-by: Sami Tolvanen Signed-off-by: Francisco Franco

first commit

2016-08-15T02:19:42+00:00

xavi/android_kernel_m2note/drivers/md/dm-bufio.c, branch ng-7.1.2

BACKPORT: dm bufio: don't take the lock in dm_bufio_shrink_count

BACKPORT: dm bufio: avoid sleeping while holding the dm_bufio lock

first commit