linux-kernel - Re: [patch v2] mm, oom: normalize oom scores to oom_score

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Date:	Wed, 23 May 2012 23:02:10 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Dave Jones <davej@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch v2] mm, oom: normalize oom scores to oom_score_adj scale
 only for userspace

On Wed, 23 May 2012, Andrew Morton wrote:

> > @@ -367,12 +354,13 @@ static struct task_struct *select_bad_process(unsigned int *ppoints,
> >  		}
> >  
> >  		points = oom_badness(p, memcg, nodemask, totalpages);
> > -		if (points > *ppoints) {
> > +		if (points > chosen_points) {
> >  			chosen = p;
> > -			*ppoints = points;
> > +			chosen_points = points;
> >  		}
> >  	} while_each_thread(g, p);
> >  
> > +	*ppoints = chosen_points * 1000 / totalpages;
> >  	return chosen;
> >  }
> >  
> 
> It's still not obvious that we always avoid the divide-by-zero here. 
> If there's some weird way of convincing constrained_alloc() to look at
> an empty nodemask, or a nodemask which covers only empty nodes then
> blam.
> 
> Now, it's probably the case that this is a can't-happen but that
> guarantee would be pretty convoluted and fragile?
> 

It can only happen for memcg with a zero limit, something I tried to 
prevent by not allowing tasks to be attached to the memcgs with such a 
limit in a different patch but you didn't like that :)

So I fixed it in this patch with this:

@@ -572,7 +560,7 @@ void mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
 	}
 
 	check_panic_on_oom(CONSTRAINT_MEMCG, gfp_mask, order, NULL);
-	limit = mem_cgroup_get_limit(memcg) >> PAGE_SHIFT;
+	limit = mem_cgroup_get_limit(memcg) >> PAGE_SHIFT ? : 1;
 	read_lock(&tasklist_lock);
 	p = select_bad_process(&points, limit, memcg, NULL, false);
 	if (p && PTR_ERR(p) != -1UL)

Cpusets do not allow threads to be attached without a set of mems or the 
final mem in a cpuset to be removed while tasks are still attached.  The 
page allocator certainly wouldn't be calling the oom killer for a set of 
zones that span no pages.

Any suggestion on where to put the check for !totalpages so it's easier to 
understand?
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