linux-kernel - Re: [PATCH v2 0/4] memcg: Low-limit reclaim

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Date:	Wed, 30 Apr 2014 18:49:14 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Michal Hocko <mhocko@...e.cz>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Greg Thelen <gthelen@...gle.com>,
	Michel Lespinasse <walken@...gle.com>,
	Tejun Heo <tj@...nel.org>, Hugh Dickins <hughd@...gle.com>,
	Roman Gushchin <klamm@...dex-team.ru>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH v2 0/4] memcg: Low-limit reclaim

On Wed, Apr 30, 2014 at 02:52:38PM -0700, Andrew Morton wrote:
> On Mon, 28 Apr 2014 14:26:41 +0200 Michal Hocko <mhocko@...e.cz> wrote:
> 
> > Hi,
> > previous discussions have shown that soft limits cannot be reformed
> > (http://lwn.net/Articles/555249/). This series introduces an alternative
> > approach for protecting memory allocated to processes executing within
> > a memory cgroup controller. It is based on a new tunable that was
> > discussed with Johannes and Tejun held during the kernel summit 2013 and
> > at LSF 2014.
> > 
> > This patchset introduces such low limit that is functionally similar
> > to a minimum guarantee. Memcgs which are under their lowlimit are not
> > considered eligible for the reclaim (both global and hardlimit) unless
> > all groups under the reclaimed hierarchy are below the low limit when
> > all of them are considered eligible.
> 
> Permitting containers to avoid global reclaim sounds rather worrisome.
> 
> Fairness: won't it permit processes to completely protect their memory
> while everything else in the system is getting utterly pounded?  We
> need to consider global-vs-memcg fairness as well as memcg-vs-memgc.

Yes.

> Security: can this feature be used to DoS the machine?  Set up enough
> hierarchies which are below their low limit and we risk memory
> exhaustion and swap-thrashing and oom-killings for other processes.

And yes.

However, setting the low limit is a priviliged operation, so I don't
see how you could do worse with it than with mlock, disabling swap
etc.
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