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

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Date:	Mon, 29 Oct 2012 15:15:34 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Glauber Costa <glommer@...allels.com>
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

On Mon 29-10-12 17:58:45, Glauber Costa wrote:
> 
> > 
> > Changes since v1
> > - use kerndoc
> > - be more specific about mem_cgroup_move_parent possible failures
> > 
> > Signed-off-by: Michal Hocko <mhocko@...e.cz>
> > Reviewed-by: Tejun Heo <tj@...nel.org>
> Reviewed-by: Glauber Costa <glommer@...allels.com>

Thanks!

> > + * 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?)

-- 
Michal Hocko
SUSE Labs
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