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 10:34:01 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Michal Hocko <mhocko@...nel.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, Aug 15, 2016 at 04:04:39PM +0200, Michal Hocko wrote:
> On Mon 15-08-16 09:37:48, Johannes Weiner wrote:
> > On Mon, Aug 15, 2016 at 02:46:19PM +0200, Michal Hocko wrote:
> > > 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?
> > 
> > Page cache shadow entries refer to these IDs weakly. It's possible to
> > refault with a recently recycled memcg ID and crash. That's why we do
> > the whole alloc(NULL) -> replace(memcg) dance in the first place.
> 
> Ahh, OK, you are right. So I have moved the idr_replace into
> mem_cgroup_css_alloc. Does the following incremental diff looks better?

Yep, looks good to me. Thanks!