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

[Li Zefan <lizefan@huawei.com>]

On 2013/4/3 15:43, Michal Hocko wrote:
> On Wed 03-04-13 11:49:29, Li Zefan wrote:
>>>> 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.
>>> ---
>>> >From 3aff5d958f1d0717795018f7d0d6b63d53ad1dd3 Mon Sep 17 00:00:00 2001
>>> From: Li Zefan <lizefan@huawei.com>
>>> Date: Tue, 2 Apr 2013 16:37:39 +0200
>>> Subject: [PATCH] memcg: don't do cleanup manually if mem_cgroup_css_online()
>>>  fails
>>>
>>> mem_cgroup_css_online is called with memcg with refcnt = 1 and it
>>> expects that mem_cgroup_css_free will drop this last reference.
>>> This doesn't hold when memcg_init_kmem fails though and a reference is
>>> dropped for both memcg and its parent explicitly if it returns with an
>>> error.
>>>
>>> This is not correct for two reasons. Firstly mem_cgroup_put on parent is
>>> excessive because mem_cgroup_put is hierarchy aware and secondly only
>>> memcg_propagate_kmem takes an additional reference.
>>>
>>> The first one is a real use-after-free bug introduced by e4715f01
>>> (memcg: avoid dangling reference count in creation failure)
>>>
>>> The later one is non-issue right now because the only implementation
>>> of init_cgroup seems to be tcp_init_cgroup which doesn't fail
>>> but it is better to make the error handling saner and move the
>>> mem_cgroup_put(memcg) to memcg_propagate_kmem where it belongs.
>>>
>>> Signed-off-by: Li Zefan <lizefan@huawei.com>
>>> Signed-off-by: Michal Hocko <mhocko@suse.cz>
>>> ---
>>>  mm/memcontrol.c |   13 +++----------
>>>  1 file changed, 3 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>>> index f608546..cf9ba7e 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);
>>
>> Correct me if I'm wrong, but I think:
>>
>> When memcg_propagate_kmem() calls mem_cgroup_get(), it's because the kmemcg
>> is active by inheritance. Then when memcg_update_cache_sizes() fails, leading
>> to mem_cgroup_css_free() is called by cgroup core:
>>
>> static void mem_cgroup_css_free(struct cgroup *cont)
>> {
>>         struct mem_cgroup *memcg = mem_cgroup_from_cont(cont);
>>
>>         kmem_cgroup_destroy(memcg);
>>
>>         mem_cgroup_put(memcg);
>> }
>>
>> static void kmem_cgroup_destroy(struct mem_cgroup *memcg)
>> {
>>         mem_cgroup_sockets_destroy(memcg);
>>
>>         memcg_kmem_mark_dead(memcg);
>>
>>         if (res_counter_read_u64(&memcg->kmem, RES_USAGE) != 0)
>>                 return;
>>
>>         if (memcg_kmem_test_and_clear_dead(memcg))
>>                 mem_cgroup_put(memcg);    <------- !!!!!!!!!
>> }
> 
> But memcg_update_cache_sizes calls memcg_kmem_clear_activated on the
> error path.
> 

But memcg_kmem_mark_dead() checks the ACCOUNT flag not the ACCOUNTED flag.
Am I missing something?

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