linux-kernel - Re: [patch 7/8] mm, memcg: allow processes handling oom notifications to access reserves

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Date:	Wed, 4 Dec 2013 21:50:26 -0500
From:	Tejun Heo <tj@...nel.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Pekka Enberg <penberg@...nel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Li Zefan <lizefan@...wei.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, cgroups@...r.kernel.org
Subject: Re: [patch 7/8] mm, memcg: allow processes handling oom
 notifications to access reserves

Hello,

On Wed, Dec 04, 2013 at 05:49:04PM -0800, David Rientjes wrote:
> That's not what this series is addressing, though, and in fact it's quite 
> the opposite.  It acknowledges that userspace oom handlers need to 
> allocate and that anything else would be too difficult to maintain 
> (thereby agreeing with the above), so we must set aside memory that they 
> are exclusively allowed to access.  For the vast majority of users who 
> will not use userspace oom handlers, they can just use the default value 
> of memory.oom_reserve_in_bytes == 0 and they incur absolutely no side-
> effects as a result of this series.

Umm.. without delving into details, aren't you basically creating a
memory cgroup inside a memory cgroup?  Doesn't sound like a
particularly well thought-out plan to me.

> For those who do use userspace oom handlers, like Google, this allows us 
> to set aside memory to allow the userspace oom handlers to kill a process, 
> dump the heap, send a signal, drop caches, etc. when waking up.

Seems kinda obvious.  Put it in a separate cgroup?  You're basically
saying it doesn't want to be under the same memory limit as the
processes that it's looking over.  That's like the definition of being
in a different cgroup.

Thanks.

-- 
tejun
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