linux-kernel - Re: [PATCH stable-4.4 1/3] mm: memcontrol: fix cgroup creation failure after many small jobs

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Date:	Mon, 15 Aug 2016 14:46:19 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Stable tree <stable@...r.kernel.org>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>,
	Nikolay Borisov <kernel@...p.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH stable-4.4 1/3] mm: memcontrol: fix cgroup creation
 failure after many small jobs

On Mon 15-08-16 08:34:07, Johannes Weiner wrote:
> Hi Michal, thanks for doing this. There is only one issue I can see:
> 
> On Fri, Aug 12, 2016 at 11:56:17AM +0200, Michal Hocko wrote:
> > @@ -4171,17 +4211,27 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
> >  	if (!memcg)
> >  		return NULL;
> >  
> > +	memcg->id.id = idr_alloc(&mem_cgroup_idr, NULL,
> > +				 1, MEM_CGROUP_ID_MAX,
> > +				 GFP_KERNEL);
> > +	if (memcg->id.id < 0)
> > +		goto out_free;
> > +
> >  	memcg->stat = alloc_percpu(struct mem_cgroup_stat_cpu);
> >  	if (!memcg->stat)
> > -		goto out_free;
> > +		goto out_idr;
> >  
> >  	if (memcg_wb_domain_init(memcg, GFP_KERNEL))
> >  		goto out_free_stat;
> >  
> > +	idr_replace(&mem_cgroup_idr, memcg, memcg->id.id);
> 
> This publishes the memcg object too early. Before 4.5, the memcg is
> not fully initialized in mem_cgroup_alloc(). You have to move the
> idr_replace() down to that function (and idr_remove() on free_out).

You are right. I am just wondering whether it matters. Nobody should see
the id so nobody will be looking it up, no?

> >  	return memcg;
> >  
> >  out_free_stat:
> >  	free_percpu(memcg->stat);
> > +out_idr:
> > +	if (memcg->id.id > 0)
> > +		idr_remove(&mem_cgroup_idr, memcg->id.id);
> 
> The > 0 check seems unnecessary, no?

Yes, I will drop it.

Thanks!
-- 
Michal Hocko
SUSE Labs