From: Michal Hocko <mhocko@suse.cz>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Glauber Costa <glommer@parallels.com>,
linux-mm@kvack.org, cgroups@vger.kernel.org,
linux-kernel@vger.kernel.org, Tejun Heo <tj@kernel.org>,
Li Zefan <lizefan@huawei.com>,
Johannes Weiner <hannes@cmpxchg.org>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
Balbir Singh <bsingharora@gmail.com>
Subject: Re: [PATCH v3 3/6] memcg: Simplify mem_cgroup_force_empty_list error handling
Date: Tue, 30 Oct 2012 11:35:59 +0100 [thread overview]
Message-ID: <20121030103559.GA7394@dhcp22.suse.cz> (raw)
In-Reply-To: <20121029150022.a595b866.akpm@linux-foundation.org>
On Mon 29-10-12 15:00:22, Andrew Morton wrote:
> On Mon, 29 Oct 2012 17:58:45 +0400
> Glauber Costa <glommer@parallels.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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next prev parent reply other threads:[~2012-10-30 10:36 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-10-26 11:37 memcg/cgroup: do not fail fail on pre_destroy callbacks Michal Hocko
2012-10-26 11:37 ` [PATCH v3 1/6] memcg: split mem_cgroup_force_empty into reclaiming and reparenting parts Michal Hocko
2012-10-29 13:45 ` Glauber Costa
2012-10-31 16:29 ` Johannes Weiner
2012-10-26 11:37 ` [PATCH v3 2/6] memcg: root_cgroup cannot reach mem_cgroup_move_parent Michal Hocko
2012-10-29 13:48 ` Glauber Costa
2012-10-29 13:52 ` Michal Hocko
2012-10-31 16:31 ` Johannes Weiner
2012-10-26 11:37 ` [PATCH v3 3/6] memcg: Simplify mem_cgroup_force_empty_list error handling Michal Hocko
2012-10-29 13:58 ` Glauber Costa
2012-10-29 14:15 ` Michal Hocko
2012-10-29 15:09 ` Glauber Costa
2012-10-29 22:00 ` Andrew Morton
2012-10-30 10:35 ` Michal Hocko [this message]
2012-10-31 21:30 ` Andrew Morton
2012-11-13 21:10 ` Johannes Weiner
2012-11-14 13:59 ` Michal Hocko
2012-11-14 18:33 ` Johannes Weiner
2012-10-26 11:37 ` [PATCH v3 4/6] cgroups: forbid pre_destroy callback to fail Michal Hocko
2012-10-29 14:04 ` Glauber Costa
2012-10-29 14:06 ` Glauber Costa
2012-10-29 14:17 ` Michal Hocko
2012-11-13 21:13 ` Johannes Weiner
2012-10-26 11:37 ` [PATCH v3 5/6] memcg: make mem_cgroup_reparent_charges non failing Michal Hocko
2012-10-29 14:07 ` Glauber Costa
2012-10-26 11:37 ` [PATCH v3 6/6] hugetlb: do not fail in hugetlb_cgroup_pre_destroy Michal Hocko
2012-10-29 14:08 ` Glauber Costa
2012-10-29 23:26 ` memcg/cgroup: do not fail fail on pre_destroy callbacks Tejun Heo
2012-10-30 23:37 ` Michal Hocko
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