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Date: Tue, 2 Apr 2013 17:04:22 +0200
From: Michal Hocko <mhocko@suse.cz>
Subject: [PATCH -v2] memcg: don't do cleanup manually if
 mem_cgroup_css_online() fails
Message-ID: <20130402150422.GB32520@dhcp22.suse.cz>
References: <515A8A40.6020406@huawei.com>
 <20130402121600.GK24345@dhcp22.suse.cz>
 <20130402141646.GQ24345@dhcp22.suse.cz>
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To: Glauber Costa <glommer@parallels.com>
Cc: Li Zefan <lizefan@huawei.com>, Johannes Weiner <hannes@cmpxchg.org>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, LKML <linux-kernel@vger.kernel.org>, Cgroups <cgroups@vger.kernel.org>, linux-mm@kvack.org

On Tue 02-04-13 18:33:30, Glauber Costa wrote:
> On 04/02/2013 06:28 PM, Michal Hocko wrote:
> > On Tue 02-04-13 18:20:56, Glauber Costa wrote:
> >> On 04/02/2013 06:16 PM, Michal Hocko wrote:
> >>>  mem_cgroup_css_online
> >>>       memcg_init_kmem
> >>>         mem_cgroup_get		# refcnt = 2
> >>>           memcg_update_all_caches
> >>>             memcg_update_cache_size	# fails with ENOMEM
> >>
> >> Here is the thing: this one in kmem only happens for kmem enabled
> >> memcgs. For those, we tend to do a get once, and put only when the last
> >> kmem reference is gone.
> >>
> >> For non-kmem memcgs, refcnt will be 1 here, and will be balanced out by
> >> the mem_cgroup_put() in css_free.
> > 
> > So we need this, right?
> > ---
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index f608546..2ef875d 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -5306,6 +5306,8 @@ static int memcg_propagate_kmem(struct mem_cgroup *memcg)
> >  	ret = memcg_update_cache_sizes(memcg);
> >  	mutex_unlock(&set_limit_mutex);
> >  out:
> > +	if (ret)
> > +		mem_cgroup_put(memcg);
> >  	return ret;
> >  }
> >  #endif /* CONFIG_MEMCG_KMEM */
> > @@ -6417,16 +6419,6 @@ mem_cgroup_css_online(struct cgroup *cont)
> >  
> >  	error = memcg_init_kmem(memcg, &mem_cgroup_subsys);
> >  	mutex_unlock(&memcg_create_mutex);
> > -	if (error) {
> > -		/*
> > -		 * We call put now because our (and parent's) refcnts
> > -		 * are already in place. mem_cgroup_put() will internally
> > -		 * call __mem_cgroup_free, so return directly
> > -		 */
> > -		mem_cgroup_put(memcg);
> > -		if (parent->use_hierarchy)
> > -			mem_cgroup_put(parent);
> > -	}
> >  	return error;
> >  }
> >  
> > 
> Yes, indeed you are very right - and thanks for looking at such depth.

So what about the patch bellow? It seems that I provoked all this mess
but my brain managed to push it away so I do not remember why I thought
the parent needs reference drop... It is "only" 3.9 thing fortunately.
---