linux-kernel - Re: [RFC][PATCH 1/4] New css->refcnt implementation.

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Date:	Wed, 10 Dec 2008 19:23:44 +0900 (JST)
From:	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>
To:	"Paul Menage" <menage@...gle.com>
Cc:	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	"lizf@...fujitsu.com" <lizf@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 1/4] New css->refcnt implementation.

Paul Menage said:
> On Fri, Dec 5, 2008 at 3:24 AM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@...fujitsu.com> wrote:
>>> The basic rule is that you're only supposed to increment the css
>>> refcount if you have:
>>>
>>> - a reference to a task in the cgroup (that is pinned via task_lock()
>>> so it can't be moved away)
>>> or
>>> - an existing reference to the css
>>>
>> My problem is that we can do css_get() after pre_destroy() and
>> css's refcnt goes down to 0.
>
> But where are you getting the reference from in order to do css_get()?
> Which call in mem cgroup are you concerned about?
>
mem_cgroup_try_charge_swapin() at el. all swap-in codes.
In that functions, follwoing occurs.
==
    assume swp_entry which is being swapped-in
    lookup swap_cgroup for swp_entry.
    get memcg from swap_cgroup.
    charge() against memcg got by swap_cgroup. charge() will do css_get()
==
Currently, mem_cgroup->obsolete is used for making this never happen.
But, mem_cgroup->obsolete flag is broken,now.
I'm looking for alternative. (see other patches. I know there are several
ways to go.)

*AND* any kinds of hierarchy-tree-walk algorithm may call css_get() against
cgroup under rmdir() if it's not marked as REMOVED.

Thanks,
-Kame

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