linux-kernel - Re: [PATCH v3 3/6] memcg: Simplify mem_cgroup_force_empty

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Date:	Mon, 29 Oct 2012 19:09:28 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	<linux-mm@...ck.org>, <cgroups@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
	Johannes Weiner <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Balbir Singh <bsingharora@...il.com>
Subject: Re: [PATCH v3 3/6] memcg: Simplify mem_cgroup_force_empty_list error
 handling

>>> + * move charges to its parent or the root cgroup if the group has no
>>> + * parent (aka use_hierarchy==0).
>>> + * Although this might fail (get_page_unless_zero, isolate_lru_page or
>>> + * mem_cgroup_move_account fails) the failure is always temporary and
>>> + * it signals a race with a page removal/uncharge or migration. In the
>>> + * first case the page is on the way out and it will vanish from the LRU
>>> + * on the next attempt and the call should be retried later.
>>> + * Isolation from the LRU fails only if page has been isolated from
>>> + * the LRU since we looked at it and that usually means either global
>>> + * reclaim or migration going on. The page will either get back to the
>>> + * LRU or vanish.
>>
>> I just wonder for how long can it go in the worst case?
>  
> That's a good question and to be honest I have no idea. The point is
> that it will terminate eventually and that the group is on the way out
> so the time to complete the removal is not a big deal IMHO. We had
> basically similar situation previously when we would need to repeat
> rmdir loop on EBUSY. The only change is that we do not have to retry
> anymore.
> 
> So the key point is to check whether my assumption about temporarily is
> correct and that we cannot block the rest of the kernel/userspace to
> proceed even though we are waiting for finalization. I believe this is
> true but... (last famous words?)
> 
At least for me, it seems that this will hold.

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