linux-kernel - Re: [RFC][PATCH 2/7] memcg: don't use mem_cgroup

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Date:	Sun, 7 Apr 2013 11:32:48 +0800
From:	Li Zefan <lizefan@...wei.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	Glauber Costa <glommer@...allels.com>, <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [RFC][PATCH 2/7] memcg: don't use mem_cgroup_get() when creating
 a kmemcg cache

>>> You are putting references but I do not see any single css_{try}get
>>> here. /me puzzled.
>>>
>>
>> There are two things being done in this code:
>> First, we acquired a css_ref to make sure that the underlying cgroup
>> would not go away. That is a short lived reference, and it is put as
>> soon as the cache is created.
>> At this point, we acquire a long-lived per-cache memcg reference count
>> to guarantee that the memcg will still be alive.
>>
>> so it is:
>>
>> enqueue: css_get
>> create : memcg_get, css_put
>> destroy: css_put
>>
>> If I understand Li's patch correctly, he is not touching the first
>> css_get, only turning that into the long lived reference (which was not
>> possible before, since that would prevent rmdir).
>>
>> Then he only needs to get rid of the memcg_get, change the memcg_put to
>> css_put, and get rid of the now extra css_put.
>>
>> He is issuing extra css_puts in memcg_create_kmem_cache, but only in
>> failure paths. So the code reads as:
>> * css_get on enqueue (already done, so not shown in patch)
>> * if it fails, css_put
>> * if it succeeds, don't do anything. This is already the long-lived
>> reference count. put it at release time.
> 
> OK, this makes more sense now. It is __memcg_create_cache_enqueue which
> takes the reference and it is not put after this because it replaced
> mem_cgroup reference counting.
> Li, please put something along these lines into the changelog. This is
> really tricky and easy to get misunderstand.
> 

Yeah, I think I'll just steal Glauber's explanation as the changelog.

> You can put my Acked-by then.
> 

Thanks!

>> The code looks correct, and of course, extremely simpler due to the
>> use of a single reference.
>>
>> Li, am I right in my understanding that this is your intention?
>>


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