linux-kernel - Re: [PATCH 10/10] mm, oom: hide mm which is shared with kthread or global init

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Date:	Thu, 16 Jun 2016 08:31:27 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-mm@...ck.org,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	David Rientjes <rientjes@...gle.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 10/10] mm, oom: hide mm which is shared with kthread or
 global init

On Wed 15-06-16 16:37:01, Oleg Nesterov wrote:
> Michal,
> 
> I am going to ack the whole series, but send some nits/questions,
> 
> On 06/09, Michal Hocko wrote:
> >
> > @@ -283,10 +283,22 @@ enum oom_scan_t oom_scan_process_thread(struct oom_control *oc,
> >  
> >  	/*
> >  	 * This task already has access to memory reserves and is being killed.
> > -	 * Don't allow any other task to have access to the reserves.
> > +	 * Don't allow any other task to have access to the reserves unless
> > +	 * the task has MMF_OOM_REAPED because chances that it would release
> > +	 * any memory is quite low.
> >  	 */
> > -	if (!is_sysrq_oom(oc) && atomic_read(&task->signal->oom_victims))
> > -		return OOM_SCAN_ABORT;
> > +	if (!is_sysrq_oom(oc) && atomic_read(&task->signal->oom_victims)) {
> > +		struct task_struct *p = find_lock_task_mm(task);
> > +		enum oom_scan_t ret = OOM_SCAN_ABORT;
> > +
> > +		if (p) {
> > +			if (test_bit(MMF_OOM_REAPED, &p->mm->flags))
> > +				ret = OOM_SCAN_CONTINUE;
> > +			task_unlock(p);
> 
> OK, but perhaps it would be beter to change oom_badness() to return zero if
> MMF_OOM_REAPED is set?

We already do that:
	if (adj == OOM_SCORE_ADJ_MIN ||
			test_bit(MMF_OOM_REAPED, &p->mm->flags) ||
			in_vfork(p)) {
		task_unlock(p);
		return 0;
	}

It is kind of subtle that we have to check it 2 times but we would have
to rework this code much more because oom_badness only can tell to
ignore the task but not to abort scanning altogether currently. If we
should change this I would suggest a separate patch.
-- 
Michal Hocko
SUSE Labs