Re: [patch v2] mm, oom: normalize oom scores to oom_score_adj scale only for userspace

[David Rientjes <rientjes@google.com>]

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?