[PATCH 2/2] mm,oom: Use ALLOC_OOM for OOM victim's last second allocation.

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Manish Jaggi noticed that running LTP oom01/oom02 ltp tests with high core
count causes random kernel panics when an OOM victim which consumed memory
in a way the OOM reaper does not help was selected by the OOM killer [1].

----------
oom02       0  TINFO  :  start OOM testing for mlocked pages.
oom02       0  TINFO  :  expected victim is 4578.
oom02       0  TINFO  :  thread (ffff8b0e71f0), allocating 3221225472 bytes.
oom02       0  TINFO  :  thread (ffff8b8e71f0), allocating 3221225472 bytes.
(...snipped...)
oom02       0  TINFO  :  thread (ffff8a0e71f0), allocating 3221225472 bytes.
[  364.737486] oom02:4583 invoked oom-killer: gfp_mask=0x16080c0(GFP_KERNEL|__GFP_ZERO|__GFP_NOTRACK), nodemask=1,  order=0, oom_score_adj=0
(...snipped...)
[  365.036127] [ pid ]   uid  tgid total_vm      rss nr_ptes nr_pmds swapents oom_score_adj name
[  365.044691] [ 1905]     0  1905     3236     1714      10       4        0             0 systemd-journal
[  365.054172] [ 1908]     0  1908    20247      590       8       4        0             0 lvmetad
[  365.062959] [ 2421]     0  2421     3241      878       9       3        0         -1000 systemd-udevd
[  365.072266] [ 3125]     0  3125     3834      719       9       4        0         -1000 auditd
[  365.080963] [ 3145]     0  3145     1086      630       6       4        0             0 systemd-logind
[  365.090353] [ 3146]     0  3146     1208      596       7       3        0             0 irqbalance
[  365.099413] [ 3147]    81  3147     1118      625       5       4        0          -900 dbus-daemon
[  365.108548] [ 3149]   998  3149   116294     4180      26       5        0             0 polkitd
[  365.117333] [ 3164]   997  3164    19992      785       9       3        0             0 chronyd
[  365.126118] [ 3180]     0  3180    55605     7880      29       3        0             0 firewalld
[  365.135075] [ 3187]     0  3187    87842     3033      26       3        0             0 NetworkManager
[  365.144465] [ 3290]     0  3290    43037     1224      16       5        0             0 rsyslogd
[  365.153335] [ 3295]     0  3295   108279     6617      30       3        0             0 tuned
[  365.161944] [ 3308]     0  3308    27846      676      11       3        0             0 crond
[  365.170554] [ 3309]     0  3309     3332      616      10       3        0         -1000 sshd
[  365.179076] [ 3371]     0  3371    27307      364       6       3        0             0 agetty
[  365.187790] [ 3375]     0  3375    29397     1125      11       3        0             0 login
[  365.196402] [ 4178]     0  4178     4797     1119      14       4        0             0 master
[  365.205101] [ 4209]    89  4209     4823     1396      12       4        0             0 pickup
[  365.213798] [ 4211]    89  4211     4842     1485      12       3        0             0 qmgr
[  365.222325] [ 4491]     0  4491    27965     1022       8       3        0             0 bash
[  365.230849] [ 4513]     0  4513      670      365       5       3        0             0 oom02
[  365.239459] [ 4578]     0  4578 37776030 32890957   64257     138        0             0 oom02
[  365.248067] Out of memory: Kill process 4578 (oom02) score 952 or sacrifice child
[  365.255581] Killed process 4578 (oom02) total-vm:151104120kB, anon-rss:131562528kB, file-rss:1300kB, shmem-rss:0kB
[  365.266829] out_of_memory: Current (4583) has a pending SIGKILL
[  365.267347] oom_reaper: reaped process 4578 (oom02), now anon-rss:131559616kB, file-rss:0kB, shmem-rss:0kB
[  365.282658] oom_reaper: reaped process 4583 (oom02), now anon-rss:131561664kB, file-rss:0kB, shmem-rss:0kB
[  365.283361] oom02:4586 invoked oom-killer: gfp_mask=0x16040c0(GFP_KERNEL|__GFP_COMP|__GFP_NOTRACK), nodemask=1,  order=0, oom_score_adj=0
(...snipped...)
[  365.576164] oom02:4585 invoked oom-killer: gfp_mask=0x16080c0(GFP_KERNEL|__GFP_ZERO|__GFP_NOTRACK), nodemask=1,  order=0, oom_score_adj=0
(...snipped...)
[  365.576298] [ pid ]   uid  tgid total_vm      rss nr_ptes nr_pmds swapents oom_score_adj name
[  365.576338] [ 2421]     0  2421     3241      878       9       3        0         -1000 systemd-udevd
[  365.576342] [ 3125]     0  3125     3834      719       9       4        0         -1000 auditd
[  365.576347] [ 3309]     0  3309     3332      616      10       3        0         -1000 sshd
[  365.576356] [ 4580]     0  4578 37776030 32890417   64258     138        0             0 oom02
[  365.576361] Kernel panic - not syncing: Out of memory and no killable processes...
----------

Since commit 696453e66630ad45 ("mm, oom: task_will_free_mem should skip
oom_reaped tasks") changed task_will_free_mem(current) in out_of_memory()
to return false as soon as MMF_OOM_SKIP is set, many threads sharing the
victim's mm were not able to try allocation from memory reserves after the
OOM reaper gave up reclaiming memory.

Until Linux 4.7, we were using

  if (current->mm &&
      (fatal_signal_pending(current) || task_will_free_mem(current)))

as a condition to try allocation from memory reserves with the risk of OOM
lockup, but reports like [1] were impossible. Linux 4.8+ are regressed
compared to Linux 4.7 due to the risk of needlessly selecting more OOM
victims.

There is no need that the OOM victim is such malicious that consumes all
memory. It is possible that a multithreaded but non memory hog process is
selected by the OOM killer, and the OOM reaper fails to reclaim memory due
to e.g. khugepaged [2], and the process fails to try allocation from memory
reserves. Therefore, this patch allows OOM victims to use ALLOC_OOM watermark
for last second allocation attempt.

[1] http://lkml.kernel.org/r/e6c83a26-1d59-4afd-55cf-04e58bdde188@xxxxxxxxxxxxxxxxxx
[2] http://lkml.kernel.org/r/201708090835.ICI69305.VFFOLMHOStJOQF@xxxxxxxxxxxxxxxxxxx

Fixes: 696453e66630ad45 ("mm, oom: task_will_free_mem should skip oom_reaped tasks")
Reported-by: Manish Jaggi <mjaggi@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
---
 mm/page_alloc.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6654f52..382ed57 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4112,9 +4112,14 @@ struct page *alloc_pages_before_oomkill(const struct oom_control *oc)
 	 * we're still under heavy pressure. But make sure that this reclaim
 	 * attempt shall not depend on __GFP_DIRECT_RECLAIM && !__GFP_NORETRY
 	 * allocation which will never fail due to oom_lock already held.
+	 * Also, make sure that OOM victims can try ALLOC_OOM watermark in case
+	 * they haven't tried ALLOC_OOM watermark.
 	 */
 	return get_page_from_freelist((oc->gfp_mask | __GFP_HARDWALL) &
 				      ~__GFP_DIRECT_RECLAIM, oc->order,
+				      oom_reserves_allowed(current) &&
+				      !(oc->gfp_mask & __GFP_NOMEMALLOC) ?
+				      ALLOC_OOM :
 				      ALLOC_WMARK_HIGH|ALLOC_CPUSET, oc->ac);
 }
 
-- 
1.8.3.1

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