linux-kernel - Re: [PATCH -mm 2/4] oom: select_bad_process: PF_EXITING check should take ->mm into account

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Date:	Tue, 6 Apr 2010 14:18:11 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	anfei <anfei.zhou@...il.com>
Cc:	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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 -mm 2/4] oom: select_bad_process: PF_EXITING check
	should take ->mm into account

On 04/06, anfei wrote:
>
> On Fri, Apr 02, 2010 at 08:32:16PM +0200, Oleg Nesterov wrote:
> > select_bad_process() checks PF_EXITING to detect the task which
> > is going to release its memory, but the logic is very wrong.
> >
> > 	- a single process P with the dead group leader disables
> > 	  select_bad_process() completely, it will always return
> > 	  ERR_PTR() while P can live forever
> >
> > 	- if the PF_EXITING task has already released its ->mm
> > 	  it doesn't make sense to expect it is goiing to free
> > 	  more memory (except task_struct/etc)
> >
> > Change the code to ignore the PF_EXITING tasks without ->mm.
> >
> > Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> > ---
> >
> >  mm/oom_kill.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > --- MM/mm/oom_kill.c~2_FIX_PF_EXITING	2010-04-02 18:51:05.000000000 +0200
> > +++ MM/mm/oom_kill.c	2010-04-02 18:58:37.000000000 +0200
> > @@ -322,7 +322,7 @@ static struct task_struct *select_bad_pr
> >  		 * the process of exiting and releasing its resources.
> >  		 * Otherwise we could get an easy OOM deadlock.
> >  		 */
> > -		if (p->flags & PF_EXITING) {
> > +		if ((p->flags & PF_EXITING) && p->mm) {
>
> Even this check is satisfied, it still can't say p is a good victim or
> it will release memory automatically if multi threaded, as the exiting
> of p doesn't mean the other threads are going to exit, so the ->mm won't
> be released.

Yes, completely agreed.

Unfortunately I forgot to copy this into the changelog, but when I
discussed this change I mentioned "still not perfect, but much better".

I do not really know what is the "right" solution. Even if we fix this
check for mt case, we also have CLONE_VM tasks.

So, this patch just tries to improve things, to avoid the easy-to-trigger
false positives.

Oleg.

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