linux-kernel - Re: [patch v4] mm, oom: fix unnecessary killing of additional processes

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Message-ID: <f2e09fbf-700c-19d6-7dd8-42683507b5d1@i-love.sakura.ne.jp>
Date:   Wed, 25 Jul 2018 07:55:51 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [patch v4] mm, oom: fix unnecessary killing of additional
 processes

On 2018/07/25 7:51, David Rientjes wrote:
> On Wed, 25 Jul 2018, Tetsuo Handa wrote:
> 
>>>> You might worry about situations where __oom_reap_task_mm() is a no-op.
>>>> But that is not always true. There is no point with emitting
>>>>
>>>>   pr_info("oom_reaper: unable to reap pid:%d (%s)\n", ...);
>>>>   debug_show_all_locks();
>>>>
>>>> noise and doing
>>>>
>>>>   set_bit(MMF_OOM_SKIP, &mm->flags);
>>>>
>>>> because exit_mmap() will not release oom_lock until __oom_reap_task_mm()
>>>> completes. That is, except extra noise, there is no difference with
>>>> current behavior which sets set_bit(MMF_OOM_SKIP, &mm->flags) after
>>>> returning from __oom_reap_task_mm().
>>>>
>>>
>>> v5 has restructured how exit_mmap() serializes its unmapping with the oom 
>>> reaper.  It sets MMF_OOM_SKIP while holding mm->mmap_sem.
>>>
>>
>> I think that v5 is still wrong. exit_mmap() keeps mmap_sem held for write does
>> not prevent oom_reap_task() from emitting the noise and setting MMF_OOM_SKIP
>> after timeout. Since your purpose is to wait for release of memory which could
>> not be reclaimed by __oom_reap_task_mm(), what if __oom_reap_task_mm() was no-op and
>> exit_mmap() was preempted immediately after returning from __oom_reap_task_mm() ?
>>
> 
> If exit_mmap() gets preempted indefinitely before it can free any memory, 
> we are better off oom killing another process.  The purpose of the timeout 
> is to give an oom victim an amount of time to free its memory and exit 
> before selecting another victim.
> 

There is no point with emitting the noise.