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Date:   Wed, 1 Jun 2022 07:22:05 -0700
From:   Roman Gushchin <roman.gushchin@linux.dev>
To:     Michal Hocko <mhocko@suse.com>
Cc:     Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>,
        Vasily Averin <vvs@openvz.org>,
        Andrew Morton <akpm@linux-foundation.org>, kernel@openvz.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Shakeel Butt <shakeelb@google.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Muchun Song <songmuchun@bytedance.com>, cgroups@vger.kernel.org
Subject: Re: [PATCH mm v3 0/9] memcg: accounting for objects allocated by
 mkdir cgroup
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On Wed, Jun 01, 2022 at 03:05:34PM +0200, Michal Hocko wrote:
> On Wed 01-06-22 11:32:26, Michal Hocko wrote:
> > On Wed 01-06-22 11:15:43, Michal Koutny wrote:
> > > On Wed, Jun 01, 2022 at 06:43:27AM +0300, Vasily Averin <vvs@openvz.org> wrote:
> > > > CT-901 /# cat /sys/fs/cgroup/memory/cgroup.subgroups_limit 
> > > > 512
> > > > CT-901 /# echo 3333 > /sys/fs/cgroup/memory/cgroup.subgroups_limit 
> > > > -bash: echo: write error: Operation not permitted
> > > > CT-901 /# echo 333 > /sys/fs/cgroup/memory/cgroup.subgroups_limit 
> > > > -bash: echo: write error: Operation not permitted
> > > > 
> > > > I doubt this way can be accepted in upstream, however for OpenVz
> > > > something like this it is mandatory because it much better
> > > > than nothing.
> > > 
> > > Is this customization of yours something like cgroup.max.descendants on
> > > the unified (v2) hierarchy? (Just curious.)
> > > 
> > > (It can be made inaccessible from within the subtree either with cgroup
> > > ns or good old FS permissions.)
> > 
> > So we already do have a limit to prevent somebody from running away with
> > the number of cgroups. Nice!

Yes, we do!

> > I was not aware of that and I guess this
> > looks like the right thing to do. So do we need more control and
> > accounting that this?
> 
> I have checked the actual implementation and noticed that cgroups are
> uncharged when offlined (rmdir-ed) which means that an adversary could
> still trick the limit and runaway while still consuming resources.
> 
> Roman, I guess the reason for this implementation was to avoid limit to
> trigger on setups with memcgs which can take quite some time to die?
> Would it make sense to make the implementation more strict to really act
> as gate against potential cgroups count runways?

The reasoning was that in many cases a user can't do much about dying cgroups,
so it's not clear how they should/would handle getting -EAGAIN on creating a
new cgroup (retrying will not help, obviously). Live cgroups can be easily
deleted, dying cgroups - not always.

I'm not sure about switching the semantics. I'd wait till Muchun's lru page
reparenting will be landed (could be within 1-2 releases, I guess) and then we
can check whether the whole problem is mostly gone. Honestly, I think we might
need to fix few another things, but it might be not that hard (in comparison
to what we already did).