linux-kernel - Re: [PATCH] oom killer: break from infinite loop

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Date:	Mon, 29 Mar 2010 13:01:58 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	anfei <anfei.zhou@...il.com>
cc:	Oleg Nesterov <oleg@...hat.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] oom killer: break from infinite loop

On Mon, 29 Mar 2010, anfei wrote:

> I think this method is okay, but it's easy to trigger another bug of
> oom.  See select_bad_process():
> 	if (!p->mm)
> 		continue;
> !p->mm is not always an unaccepted condition.  e.g. "p" is killed and
> doing exit, setting tsk->mm to NULL is before releasing the memory.
> And in multi threading environment, this happens much more.
> In __out_of_memory(), it panics if select_bad_process returns NULL.
> The simple way to fix it is as mem_cgroup_out_of_memory() does.
> 

This is fixed by 
oom-avoid-race-for-oom-killed-tasks-detaching-mm-prior-to-exit.patch in 
the -mm tree.

See 
http://userweb.kernel.org/~akpm/mmotm/broken-out/oom-avoid-race-for-oom-killed-tasks-detaching-mm-prior-to-exit.patch

> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index afeab2a..9aae208 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -588,12 +588,8 @@ retry:
>  	if (PTR_ERR(p) == -1UL)
>  		return;
>  
> -	/* Found nothing?!?! Either we hang forever, or we panic. */
> -	if (!p) {
> -		read_unlock(&tasklist_lock);
> -		dump_header(NULL, gfp_mask, order, NULL);
> -		panic("Out of memory and no killable processes...\n");
> -	}
> +	if (!p)
> +		p = current;
>  
>  	if (oom_kill_process(p, gfp_mask, order, points, NULL,
>  			     "Out of memory"))

The reason p wasn't selected is because it fails to meet the criteria for 
candidacy in select_bad_process(), not necessarily because of a race with 
the !p->mm check that the -mm patch cited above fixes.  It's quite 
possible that current has an oom_adj value of OOM_DISABLE, for example, 
where this would be wrong.
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