Date:   Fri, 28 Jul 2017 18:45:53 +0530
From:   Manish Jaggi <mjaggi@...iumnetworks.com>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: Possible race condition in oom-killer

Hello Tetsuo Handa,

On 7/28/2017 6:29 PM, Tetsuo Handa wrote:
> (Oops. Forgot to add CC.)
>
> On 2017/07/28 21:32, Michal Hocko wrote:
>> [CC linux-mm]
>>
>> On Fri 28-07-17 17:22:25, Manish Jaggi wrote:
>>> was: Re: [PATCH] mm, oom: allow oom reaper to race with exit_mmap
>>>
>>> Hi Michal,
>>> On 7/27/2017 2:54 PM, Michal Hocko wrote:
>>>> On Thu 27-07-17 13:59:09, Manish Jaggi wrote:
>>>> [...]
>>>>> With 4.11.6 I was getting random kernel panics (Out of memory - No process left to kill),
>>>>>   when running LTP oom01 /oom02 ltp tests on our arm64 hardware with ~256G memory and high core count.
>>>>> The issue experienced was as follows
>>>>> 	that either test (oom01/oom02) selected a pid as victim and waited for the pid to be killed.
>>>>> 	that pid was marked as killed but somewhere there is a race and the process didnt get killed.
>>>>> 	and the oom01/oom02 test started killing further processes, till it panics.
>>>>> IIUC this issue is quite similar to your patch description. But applying your patch I still see the issue.
>>>>> If it is not related to this patch, can you please suggest by looking at the log, what could be preventing
>>>>> the killing of victim.
>>>>>
>>>>> Log (https://pastebin.com/hg5iXRj2)
>>>>>
>>>>> As a subtest of oom02 starts, it prints out the victim - In this case 4578
>>>>>
>>>>> oom02       0  TINFO  :  start OOM testing for mlocked pages.
>>>>> oom02       0  TINFO  :  expected victim is 4578.
>>>>>
>>>>> When oom02 thread invokes oom-killer, it did select 4578  for killing...
>>>> I will definitely have a look. Can you report it in a separate email
>>>> thread please? Are you able to reproduce with the current Linus or
>>>> linux-next trees?
>>> Yes this issue is visible with linux-next.
>> Could you provide the full kernel log from this run please? I do not
>> expect there to be much difference but just to be sure that the code I
>> am looking at matches logs.
> 4578 is consuming memory as mlocked pages. But the OOM reaper cannot reclaim
> mlocked pages (i.e. can_madv_dontneed_vma() returns false due to VM_LOCKED), can it?
>
> oom02       0  TINFO  :  start OOM testing for mlocked pages.
> oom02       0  TINFO  :  expected victim is 4578.
> [  365.267347] oom_reaper: reaped process 4578 (oom02), now anon-rss:131559616kB, file-rss:0kB, shmem-rss:0kB
>
> As a result, MMF_OOM_SKIP is set without reclaiming much memory.
> Thus, it is natural that subsequent OOM victims are selected immediately because
> almost all memory is still in use. Since 4578 is multi-threaded (isn't it?),
> it will take time to call final __mmput() because mm->users are large.
> Since there are many threads, it is possible that all OOM killable processes are
> killed before final __mmput() of 4578 (which releases mlocked pages) is called.
My setup has 95 or more cores, so the large number of cores could be the 
reason for the random failure?
>> [...]
>>>>> [  365.283361] oom02:4586 invoked oom-killer: gfp_mask=0x16040c0(GFP_KERNEL|__GFP_COMP|__GFP_NOTRACK), nodemask=1,  order=0, oom_score_adj=0
>>>> Yes because
>>>> [  365.283499] Node 1 Normal free:19500kB min:33804kB low:165916kB high:298028kB active_anon:13312kB inactive_anon:172kB active_file:0kB inactive_file:1044kB unevictable:131560064kB writepending:0kB present:134213632kB managed:132113248kB mlocked:131560064kB slab_reclaimable:5748kB slab_unreclaimable:17808kB kernel_stack:2720kB pagetables:254636kB bounce:0kB free_pcp:10476kB local_pcp:144kB free_cma:0kB
>>>>
>>>> Although we have killed and reaped oom02 process Node1 is still below
>>>> min watermark and that is why we have hit the oom killer again. It
>>>> is not immediatelly clear to me why, that would require a deeper
>>>> inspection.
>>> I have a doubt here
>>> my understanding of oom test: oom() function basically forks itself and
>>> starts n threads each thread has a loop which allocates and touches memory
>>> thus will trigger oom-killer and will kill the process. the parent process
>>> is on a wait() and will print pass/fail.
>>>
>>> So IIUC when 4578 is reaped all the child threads should be terminated,
>>> which happens in pass case (line 152)
>>> But even after being killed and reaped,  the oom killer is invoked again
>>> which doesn't seem right.
>> As I've said the OOM killer hits because the memory from Node 1 didn't
>> get freed for some reasov or got immediatally populated.
> Because of mlocked pages by multi threaded process, it will take time to
> reclaim mlocked pages.
>
>>> Could it be that the process is just marked hidden from oom including its
>>> threads, thus oom-killer continues.
>> The whole process should be killed and the OOM reaper should only mark
>> the victim oom invisible _after_ the address space has been reaped (and
>> memory freed). You said the patch from
>> http://lkml.kernel.org/r/20170724072332.31903-1-mhocko@kernel.org didn't
>> help so it shouldn't be a race with the last __mmput.
>>
>> Thanks!
>>

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