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Date: Tue, 2 Apr 2013 18:33:30 +0400
From: Glauber Costa <glommer@parallels.com>
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To: Michal Hocko <mhocko@suse.cz>
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>
Subject: Re: [PATCH] memcg: don't do cleanup manually if mem_cgroup_css_online()
 fails
References: <515A8A40.6020406@huawei.com> <20130402121600.GK24345@dhcp22.suse.cz> <20130402141646.GQ24345@dhcp22.suse.cz> <515AE948.1000704@parallels.com> <20130402142825.GA32520@dhcp22.suse.cz>
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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.