Re: [PATCH -v2] memcg: don't do cleanup manually if mem_cgroup_css_online() fails

[Glauber Costa <glommer@parallels.com>]

On 04/02/2013 07:04 PM, Michal Hocko wrote:
> 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.
> ---

Li being fine with it, I am fine with it.

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