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 19:42:35 +0800
From:	anfei <anfei.zhou@...il.com>
To:	Oleg Nesterov <oleg@...hat.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 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.

>  			if (p != current)
>  				return ERR_PTR(-1UL);
>  
> 
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