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

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Date:	Tue, 30 Mar 2010 22:29:23 +0800
From:	anfei <anfei.zhou@...il.com>
To:	David Rientjes <rientjes@...gle.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, Mar 29, 2010 at 01:01:58PM -0700, David Rientjes wrote:
> 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.

I see.  And what about changing mem_cgroup_out_of_memory() too?


diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 0cb1ca4..9e89a29 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -510,8 +510,10 @@ retry:
 	if (PTR_ERR(p) == -1UL)
 		goto out;
 
-	if (!p)
-		p = current;
+	if (!p) {
+		read_unlock(&tasklist_lock);
+		panic("Out of memory and no killable processes...\n");
+	}
 
 	if (oom_kill_process(p, gfp_mask, 0, points, limit, mem,
 				"Memory cgroup out of memory"))
--
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