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

[Andrew Morton]

On Mon, 28 Apr 2014 14:26:41 +0200 Michal Hocko <mhocko@xxxxxxx> 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.

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.


All of that being said, your statement doesn't appear to be true ;)

> +static void shrink_zone(struct zone *zone, struct scan_control *sc)
> +{
> +	if (!__shrink_zone(zone, sc, true)) {
> +		/*
> +		 * First round of reclaim didn't find anything to reclaim
> +		 * because of low limit protection so try again and ignore
> +		 * the low limit this time.
> +		 */
> +		__shrink_zone(zone, sc, false);
> +	}
>  }

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