linux-kernel - Re: [PATCH] oom: fix the unsafe proc_oom

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Date:	Wed, 31 Mar 2010 11:16:28 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	anfei <anfei.zhou@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	nishimura@....nes.nec.co.jp,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Mel Gorman <mel@....ul.ie>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] oom: fix the unsafe proc_oom_score()->badness() call

On 03/30, David Rientjes wrote:
>
> On Tue, 30 Mar 2010, Oleg Nesterov wrote:
>
> > proc_oom_score(task) have a reference to task_struct, but that is all.
> > If this task was already released before we take tasklist_lock
> >
> > 	- we can't use task->group_leader, it points to nowhere
> >
> > 	- it is not safe to call badness() even if this task is
> > 	  ->group_leader, has_intersects_mems_allowed() assumes
> > 	  it is safe to iterate over ->thread_group list.
> >
> > Add the pid_alive() check to ensure __unhash_process() was not called.
> >
> > Note: I think we shouldn't use ->group_leader, badness() should return
> > the same result for any sub-thread. However this is not true currently,
> > and I think that ->mm check and list_for_each_entry(p->children) in
> > badness are not right.
> >
>
> I think it would be better to just use task and not task->group_leader.

Sure, agreed. I preserved ->group_leader just because I didn't understand
why the current code doesn't use task. But note that pid_alive() is still
needed.

I'll check the code in -mm and resend.

Oleg.

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