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@xxxxxxxxxx> 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).
