linux-kernel - Re: [PATCH 1/6] mm, oom: do not loop over all tasks if there are no external tasks sharing mm

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Message-ID: <20160526145930.GF23675@dhcp22.suse.cz>
Date:	Thu, 26 May 2016 16:59:30 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	linux-mm@...ck.org, rientjes@...gle.com, oleg@...hat.com,
	vdavydov@...allels.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] mm, oom: do not loop over all tasks if there are no
 external tasks sharing mm

On Thu 26-05-16 23:30:06, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > index 5bb2f7698ad7..0e33e912f7e4 100644
> > --- a/mm/oom_kill.c
> > +++ b/mm/oom_kill.c
> > @@ -820,6 +820,13 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
> >  	task_unlock(victim);
> >  
> >  	/*
> > +	 * skip expensive iterations over all tasks if we know that there
> > +	 * are no users outside of threads in the same thread group
> > +	 */
> > +	if (atomic_read(&mm->mm_users) <= get_nr_threads(victim))
> > +		goto oom_reap;
> 
> Is this really safe? Isn't it possible that victim thread's thread group has
> more than atomic_read(&mm->mm_users) threads which are past exit_mm() and blocked
> at exit_task_work() which are before __exit_signal() from release_task() from
> exit_notify()?

You are right. The race window between exit_mm and __exit_signal is
really large. I thought about == check instead but that wouldn't work
for the same reason, dang, it looked so promissing.

Scratch this patch then.

-- 
Michal Hocko
SUSE Labs