linux-kernel - Re: [PATCH] memcg: fix memcg_cache_name() to use cgroup

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Date:	Fri, 22 Mar 2013 14:25:23 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	Li Zefan <lizefan@...wei.com>, Tejun Heo <tj@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>, <linux-mm@...ck.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH] memcg: fix memcg_cache_name() to use cgroup_name()

On 03/22/2013 02:06 PM, Michal Hocko wrote:
> On Fri 22-03-13 14:03:30, Glauber Costa wrote:
>> On 03/22/2013 01:48 PM, Michal Hocko wrote:
>>> On Fri 22-03-13 13:41:40, Glauber Costa wrote:
>>>> On 03/22/2013 01:31 PM, Michal Hocko wrote:
>>>>> On Fri 22-03-13 12:22:23, Glauber Costa wrote:
>>>>>> On 03/22/2013 12:17 PM, Li Zefan wrote:
>>>>>>>> GFP_TEMPORARY groups short lived allocations but the mem cache is not
>>>>>>>>> an ideal candidate of this type of allocations..
>>>>>>>>>
>>>>>>> I'm not sure I'm following you...
>>>>>>>
>>>>>>> char *memcg_cache_name()
>>>>>>> {
>>>>>>> 	char *name = alloc();
>>>>>>> 	return name;
>>>>>>> }
>>>>>>>
>>>>>>> kmem_cache_dup()
>>>>>>> {
>>>>>>> 	name = memcg_cache_name();
>>>>>>> 	kmem_cache_create_memcg(name);
>>>>>>> 	free(name);
>>>>>>> }
>>>>>>>
>>>>>>> Isn't this a short lived allocation?
>>>>>>>
>>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> Thanks for identifying and fixing this.
>>>>>>
>>>>>> Li is right. The cache name will live long, but this is because the
>>>>>> slab/slub caches will strdup it internally. So the actual memcg
>>>>>> allocation is short lived.
>>>>>
>>>>> OK, I have totally missed that. Sorry about the confusion. Then all the
>>>>> churn around the allocation is pointless, no?
>>>>> What about:
>>>>
>>>> If we're really not concerned about stack, then yes. Even if always
>>>> running from workqueues, a PAGE_SIZEd stack variable seems risky to me.
>>>
>>> This is not on stack. It is static
>>>
>> Ah, right, I totally missed that. And then you're taking the mutex.
>>
>> But actually, you don't need to take the mutex. All calls to
>> kmem_cache_dup are protected by the memcg_cache_mutex.
> 
> Yes and I am not taking that mutex. I've just added lockdep assert to
> make sure that this still holds true.
> 
It is impressive what a busy week does to our brains...

I read the code as lockdep_assert(memcg_cache_mutex), and then later on
mutex_lock(&memcg_mutex). But reading again, that was a just an
rcu_read_lock(). Good thing it is Friday

You guys can add my Acked-by, and thanks again

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