linux-kernel - Re: [PATCH v2 1/3] mm: memcontrol: fix swap counter leak on swapout from offline cgroup

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Message-ID: <20160803095049.GG13263@esperanza>
Date:	Wed, 3 Aug 2016 12:50:49 +0300
From:	Vladimir Davydov <vdavydov@...tuozzo.com>
To:	Michal Hocko <mhocko@...nel.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	<stable@...r.kernel.org>, Johannes Weiner <hannes@...xchg.org>,
	<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] mm: memcontrol: fix swap counter leak on swapout
 from offline cgroup

On Tue, Aug 02, 2016 at 06:00:26PM +0200, Michal Hocko wrote:
> On Tue 02-08-16 18:00:48, Vladimir Davydov wrote:
...
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 3be791afd372..4ae12effe347 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -4036,6 +4036,24 @@ static void mem_cgroup_id_get(struct mem_cgroup *memcg)
> >  	atomic_inc(&memcg->id.ref);
> >  }
> >  
> > +static struct mem_cgroup *mem_cgroup_id_get_active(struct mem_cgroup *memcg)
> > +{
> > +	while (!atomic_inc_not_zero(&memcg->id.ref)) {
> > +		/*
> > +		 * The root cgroup cannot be destroyed, so it's refcount must
> > +		 * always be >= 1.
> > +		 */
> > +		if (memcg == root_mem_cgroup) {
> > +			VM_BUG_ON(1);
> > +			break;
> > +		}
> 
> why not simply VM_BUG_ON(memcg == root_mem_cgroup)?

Because with DEBUG_VM disabled we could wind up looping forever here if
the refcount of the root_mem_cgroup got screwed up. On production
kernels, it's better to break the loop and carry on closing eyes on
diverging counters rather than getting a lockup.