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Date: Thu, 21 Jun 2012 17:26:11 +0900
From: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
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To: Michal Hocko <mhocko@suse.cz>
CC: linux-mm <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        cgroups@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [PATCH 1/2] memcg: remove -EINTR at rmdir()
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(2012/06/19 21:40), Michal Hocko wrote:
> On Tue 19-06-12 09:09:47, KAMEZAWA Hiroyuki wrote:
>> (2012/06/18 22:30), Michal Hocko wrote:
>>> On Mon 18-06-12 20:57:23, KAMEZAWA Hiroyuki wrote:
>>>> 2 follow-up patches for "memcg: move charges to root cgroup if use_hierarchy=0",
>>>> developped/tested onto memcg-devel tree. Maybe no HUNK with -next and -mm....
>>>> -Kame
>>>> ==
>>>> memcg: remove -EINTR at rmdir()
>>>>
>>>> By commit "memcg: move charges to root cgroup if use_hierarchy=0",
>>>> no memory reclaiming will occur at removing memory cgroup.
>>>
>>> OK, so the there are only 2 reasons why move_parent could fail in this
>>> path. 1) it races with somebody else who is uncharging or moving the
>>> charge and 2) THP split.
>>> 1) works for us and 2) doens't seem to be serious enough to expect that
>>> it would stall rmdir on the group for unbound amount of time so the
>>> change is safe (can we make this into the changelog please?).
>>>
>>
>> Yes. But the failure of move_parent() (-EBUSY) will be retried.
>>
>> Remaining problems are
>>   - attaching task while pre_destroy() is called.
>>   - creating child cgroup while pre_destroy() is called.
>
> I don't know why but I thought that tasks and subgroups are not alowed
> when pre_destroy is called. If this is possible then we probably want to
> check for pending signals or at least add cond_resched.


Now, pre_destroy() call is done as

	lock_cgroup_mutex();
	do some pre-check, no child, no tasks.
	unlock_cgroup_mutex();

	->pre_destroy()

	lock_cgroup_mutex()
	check css's refcnt....

What I take care of now is following case.
		CPU A			    CPU-B
	unlock_cgroup_mutex()
	->pre_destroy()

	<delay by something>		attach new task
					add new charge
					detach the task
	lock_cgroup_mutex()
	check rss' refcnt

This will cause account leak even if I think this will not happen in the real world.
I'd like to disable attach task.

Now, our ->pre_destroy() is quite fast because we don't have no memory reclaim.
I believe we can call ->pre_destroy() without dropping cgroup_mutex.

	lock_cgroup_mutex()
	do pre-check

	->pre_destroy()

	check css's refcnt

I think this is straightforward. I'd like to post a patch.
Thanks,
-Kame






















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