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SubjectRe: [patch v4] mm, oom: fix unnecessary killing of additional processes
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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.

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