linux-kernel - Re: [PATCH] memcg: don't do cleanup manually if mem_cgroup_css

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Date:	Tue, 2 Apr 2013 16:16:46 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Li Zefan <lizefan@...wei.com>
Cc:	Glauber Costa <glommer@...allels.com>,
	Johannes Weiner <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH] memcg: don't do cleanup manually if
 mem_cgroup_css_online() fails

On Tue 02-04-13 14:16:00, Michal Hocko wrote:
> On Tue 02-04-13 15:35:28, Li Zefan wrote:
> [...]
> > @@ -6247,16 +6247,7 @@ 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;
> >  }
> 
> The mem_cgroup_put(parent) part is incorrect because mem_cgroup_put goes
> up the hierarchy already but I do not think mem_cgroup_put(memcg) should
> go away as well. Who is going to free the last reference then?
> 
> Maybe I am missing something but we have:

OK, I was missing something but "there is one reference without put"
still holds...

cgroup_create
  css = ss->css_alloc(cgrp)
    mem_cgroup_css_alloc
      atomic_set(&memcg->refcnt, 1)
  online_css(ss, cgrp)
    mem_cgroup_css_online
      memcg_init_kmem
        mem_cgroup_get		# refcnt = 2
          memcg_update_all_caches
            memcg_update_cache_size	# fails with ENOMEM
  goto err_destroy
err_destroy:
  cgroup_destroy_locked(cgrp)
    offline_css
      mem_cgroup_css_offline


There is one mem_cgroup_put from mem_cgroup_css_free from cgroup_diput
but besides that I do not see any put after the patch is applied. So I
think you really need to drop only the mem_cgroup_put on parent part.
-- 
Michal Hocko
SUSE Labs
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