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Date: Tue, 2 Aug 2016 17:51:21 +0200
From: Michal Hocko <mhocko@kernel.org>
To: Vladimir Davydov <vdavydov@virtuozzo.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] mm: memcontrol: add sanity checks for memcg->id.ref
 on get/put
Message-ID: <20160802155120.GA28900@dhcp22.suse.cz>
References: <01cbe4d1a9fd9bbd42c95e91694d8ed9c9fc2208.1470057819.git.vdavydov@virtuozzo.com>
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On Tue 02-08-16 14:45:03, Michal Hocko wrote:
> On Mon 01-08-16 16:26:26, Vladimir Davydov wrote:
> [...]
> >  static struct mem_cgroup *mem_cgroup_id_get_active(struct mem_cgroup *memcg)
> >  {
> > -	while (!atomic_inc_not_zero(&memcg->id.ref))
> > +	while (!atomic_inc_not_zero(&memcg->id.ref)) {
> > +		VM_BUG_ON(mem_cgroup_is_root(memcg));
> >  		memcg = parent_mem_cgroup(memcg);
> > +	}
> 
> why is this BUG_ON ok? What if the root.use_hierarchy=true?

Scratch that. I got it. root will never get its count down to 0. Sorry
about the noise.
-- 
Michal Hocko
SUSE Labs