linux-kernel - Re: [v5 2/4] mm, oom: cgroup-aware OOM killer

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Message-ID: <20170821094656.GA13899@castle.dhcp.TheFacebook.com>
Date:   Mon, 21 Aug 2017 10:46:56 +0100
From:   Roman Gushchin <guro@...com>
To:     David Rientjes <rientjes@...gle.com>
CC:     <linux-mm@...ck.org>, Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Tejun Heo <tj@...nel.org>, <kernel-team@...com>,
        <cgroups@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [v5 2/4] mm, oom: cgroup-aware OOM killer

On Sun, Aug 20, 2017 at 05:50:27PM -0700, David Rientjes wrote:
> On Wed, 16 Aug 2017, Roman Gushchin wrote:
> 
> > It's natural to expect that inside a container there are their own sshd,
> > "activity manager" or some other stuff, which can play with oom_score_adj.
> > If it can override the upper cgroup-level settings, the whole delegation model
> > is broken.
> > 
> 
> I don't think any delegation model related to core cgroups or memory 
> cgroup is broken, I think it's based on how memory.oom_kill_all_tasks is 
> defined.  It could very well behave as memory.oom_kill_all_eligible_tasks 
> when enacted upon.
> 
> > You can think about the oom_kill_all_tasks like the panic_on_oom,
> > but on a cgroup level. It should _guarantee_, that in case of oom
> > the whole cgroup will be destroyed completely, and will not remain
> > in a non-consistent state.
> > 
> 
> Only CAP_SYS_ADMIN has this ability to set /proc/pid/oom_score_adj to

CAP_SYS_RESOURCE

> OOM_SCORE_ADJ_MIN, so it preserves the ability to change that setting, if 
> needed, when it sets memory.oom_kill_all_tasks.  If a user gains 
> permissions to change memory.oom_kill_all_tasks, I disagree it should 
> override the CAP_SYS_ADMIN setting of /proc/pid/oom_score_adj.
> 
> I would prefer not to exclude oom disabled processes to their own sibling 
> cgroups because they would require their own reservation with cgroup v2 
> and it makes the single hierarchy model much more difficult to arrange 
> alongside cpusets, for example.
> 
> > The model you're describing is based on a trust given to these oom-unkillable
> > processes on system level. But we can't really trust some unknown processes
> > inside a cgroup that they will be able to do some useful work and finish
> > in a reasonable time; especially in case of a global memory shortage.
> 
> Yes, we prefer to panic instead of sshd, for example, being oom killed.
> We trust that sshd, as well as our own activity manager and security 
> daemons are trusted to do useful work and that we never want the kernel to 
> do this.  I'm not sure why you are describing processes that CAP_SYS_ADMIN 
> has set to be oom disabled as unknown processes.
> 
> I'd be interested in hearing the opinions of others related to a per-memcg 
> knob being allowed to override the setting of the sysadmin.

Sure, me too.

Thanks!