linux-kernel - Re: [patch -mm] mm, oom: remove oom_lock from exit

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Date:   Sat, 14 Jul 2018 06:18:58 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Michal Hocko <mhocko@...nel.org>,
        David Rientjes <rientjes@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [patch -mm] mm, oom: remove oom_lock from exit_mmap

On 2018/07/13 23:26, Michal Hocko wrote:
> On Thu 12-07-18 14:34:00, David Rientjes wrote:
> [...]
>> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
>> index 0fe4087d5151..e6328cef090f 100644
>> --- a/mm/oom_kill.c
>> +++ b/mm/oom_kill.c
>> @@ -488,9 +488,11 @@ void __oom_reap_task_mm(struct mm_struct *mm)
>>  	 * Tell all users of get_user/copy_from_user etc... that the content
>>  	 * is no longer stable. No barriers really needed because unmapping
>>  	 * should imply barriers already and the reader would hit a page fault
>> -	 * if it stumbled over a reaped memory.
>> +	 * if it stumbled over a reaped memory. If MMF_UNSTABLE is already set,
>> +	 * reaping as already occurred so nothing left to do.
>>  	 */
>> -	set_bit(MMF_UNSTABLE, &mm->flags);
>> +	if (test_and_set_bit(MMF_UNSTABLE, &mm->flags))
>> +		return;
> 
> This could lead to pre mature oom victim selection
> oom_reaper			exiting victim
> oom_reap_task			exit_mmap
>   __oom_reap_task_mm		  __oom_reap_task_mm
> 				    test_and_set_bit(MMF_UNSTABLE) # wins the race
>   test_and_set_bit(MMF_UNSTABLE)
> set_bit(MMF_OOM_SKIP) # new victim can be selected now.
> 
> Besides that, why should we back off in the first place. We can
> race the two without any problems AFAICS. We already do have proper
> synchronization between the two due to mmap_sem and MMF_OOM_SKIP.
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index fc41c0543d7f..4642964f7741 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -3073,9 +3073,7 @@ void exit_mmap(struct mm_struct *mm)
>  		 * which clears VM_LOCKED, otherwise the oom reaper cannot
>  		 * reliably test it.
>  		 */
> -		mutex_lock(&oom_lock);
>  		__oom_reap_task_mm(mm);
> -		mutex_unlock(&oom_lock);
>  
>  		set_bit(MMF_OOM_SKIP, &mm->flags);

David and Michal are using different version as a baseline here.
David is making changes using timeout based back off (in linux-next.git)
which is inappropriately trying to use MMF_UNSTABLE for two purposes.

Michal is making changes using current code (in linux.git) which does not
address David's concern.

My version ( https://marc.info/?l=linux-mm&m=153119509215026 ) is
making changes using current code which also provides oom-badness
based back off in order to address David's concern.

>  		down_write(&mm->mmap_sem);

Anyway, I suggest doing

  mutex_lock(&oom_lock);
  set_bit(MMF_OOM_SKIP, &mm->flags);
  mutex_unlock(&oom_lock);

like I mentioned at
http://lkml.kernel.org/r/201807130620.w6D6KiAJ093010@www262.sakura.ne.jp
even if we make changes on top of linux-next's timeout based back off.