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

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Message-ID: <514C2C72.5090402@parallels.com>
Date:	Fri, 22 Mar 2013 14:03:30 +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 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. So you should be
able to just use the buffer without further problems.

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