linux-kernel - Re: [PATCH] memcg: change behavior of moving charges at task move

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Date:	Thu, 22 Mar 2012 14:29:41 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	"cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
	Hugh Dickins <hughd@...gle.com>,
	"n-horiguchi@...jp.nec.com" <n-horiguchi@...jp.nec.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Glauber Costa <glommer@...allels.com>
Subject: Re: [PATCH] memcg: change behavior of moving charges at task move

On Wed, 21 Mar 2012 18:52:04 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:

> As discussed before, I post this to fix the spec and implementation of task moving.
> Then, do you think what target kernel version should be ? 3.4/3.5 ?
> but yes, it may be late for 3.4....

Well, the key information here is "what effect does the bug have upon
users".

> In documentation, it's said that 'shared anon are not moved'.
> But in implementation, the check was wrong.
> 
>   if (!move_anon() || page_mapcount(page) > 2)
> 
> Ah, memcg has been moving shared anon pages for a long time.
> 
> Then, here is a discussion about handling of shared anon pages.
> 
>  - It's complex
>  - Now, shared file caches are moved in force.
>  - It adds unclear check as page_mapcount(). To do correct check,
>    we should check swap users, etc.
>  - No one notice this implementation behavior. So, no one get benefit
>    from the design.
>  - In general, once task is moved to a cgroup for running, it will not
>    be moved....
>  - Finally, we have control knob as memory.move_charge_at_immigrate.
> 
> Here is a patch to allow moving shared pages, completely. This makes
> memcg simpler and fix current broken code.
> 
> Note:
>  IIUC, libcgroup's cgroup daemon moves tasks after exec().
>  So, it's not affected.
>  libcgroup's command "cgexec" does move itsef to a memcg and call exec()
>  without fork(). it's not affected.
> 
> Changelog:
>  - fixed PageAnon() check.
>  - remove call of lookup_swap_cache()
>  - fixed Documentation.

But you forgot to tell us :(

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