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

Glauber Costa
2013-04-03T08:08:43Z

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.