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

Johannes Weiner
2014-04-30T22:49:34Z

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@suse.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.