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

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Date:	Tue, 2 Apr 2013 16:34:33 +0800
From:	Li Zefan <lizefan@...wei.com>
To:	Glauber Costa <glommer@...allels.com>
CC:	Michal Hocko <mhocko@...e.cz>,
	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 2013/4/2 16:07, Glauber Costa wrote:
> On 04/02/2013 12:03 PM, Li Zefan wrote:
>> On 2013/4/2 15:35, Li Zefan wrote:
>>> If memcg_init_kmem() returns -errno when a memcg is being created,
>>> mem_cgroup_css_online() will decrement memcg and its parent's refcnt,
>>
>>> (but strangely there's no mem_cgroup_put() for mem_cgroup_get() called
>>> in memcg_propagate_kmem()).
>>
>> The comment in memcg_propagate_kmem() suggests it knows mem_cgroup_css_free()
>> will be called in failure, while mem_cgroup_css_online() doesn't know.
>>
> This is a bit suspicious. At first your analysis seems fair, but I've
> extensively tested memcg teardown process with kmemcg (and even
> uncovered some bugs at that), and it works when and how expected.
> 

Because this bug is in a failure path, and seems the only way to get into
this path is -ENOMEM.

> Also, note that this teardown code long predates kmemcg.
> 

Maybe this bug was introduced when ss->create() was changed to ss->css_alloc()
and ss->css_online(), and before that change ss->destroy() won't be called
if ss->create() failed.

> I am not saying your are wrong - on the contrary, you seem to be right,
> but I think this one needs to be handled with extra care. I will run
> some tests, take a look, and get back to you.
> 

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