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

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Message-ID: <20121030103559.GA7394@dhcp22.suse.cz>
Date:	Tue, 30 Oct 2012 11:35:59 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Glauber Costa <glommer@...allels.com>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.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 15:00:22, Andrew Morton wrote:
> On Mon, 29 Oct 2012 17:58:45 +0400
> Glauber Costa <glommer@...allels.com> wrote:
> 
> > > + * 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?
> 
> If the kernel is uniprocessor and the caller is SCHED_FIFO: ad infinitum!

You are right, if the rmdir (resp. echo > force_empty) at SCHED_FIFO
races with put_page (on a shared page) which gets preempted after
put_page_testzero and before __page_cache_release then we are screwed:

						put_page(page)
						  put_page_testzero
						  <preempted and page still on LRU>
mem_cgroup_force_empty_list
  page = list_entry(list->prev, struct page, lru);
  mem_cgroup_move_parent(page)
    get_page_unless_zero <fails>
  cond_resched() <scheduled again>

The race window is really small but it is definitely possible. I am not
happy about this state and it should be probably mentioned in the
patch description but I do not see any way around (except for hacks like
sched_setscheduler for the current which is, ehm...) and still keep
do_not_fail contract here.

Can we consider this as a corner case (it is much easier to kill a
machine with SCHED_FIFO than this anyway) or the concern is really
strong and we should come with a solution before this can get merged?

-- 
Michal Hocko
SUSE Labs
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