Re: [PATCH bpf-next 0/5] bpf: BPF specific memory allocator.

[Roman Gushchin <roman.gushchin@linux.dev>]

On Tue, Jul 12, 2022 at 11:52:11AM +0200, Michal Hocko wrote:
> On Tue 12-07-22 16:39:48, Yafang Shao wrote:
> > On Tue, Jul 12, 2022 at 3:40 PM Michal Hocko <mhocko@suse.com> wrote:
> [...]
> > > > Roman already sent reparenting fix:
> > > > https://patchwork.kernel.org/project/netdevbpf/patch/20220711162827.184743-1-roman.gushchin@linux.dev/
> > >
> > > Reparenting is nice but not a silver bullet. Consider a shallow
> > > hierarchy where the charging happens in the first level under the root
> > > memcg. Reparenting to the root is just pushing everything under the
> > > system resources category.
> > >
> > 
> > Agreed. That's why I don't like reparenting.
> > Reparenting just reparent the charged pages and then redirect the new
> > charge, but can't reparents the 'limit' of the original memcg.
> > So it is a risk if the original memcg is still being charged. We have
> > to forbid the destruction of the original memcg.

I agree, I also don't like reparenting for !kmem case. For kmem (and *maybe*
bpf maps is an exception), I don't think there is a better choice.

> yes, I was toying with an idea like that. I guess we really want a
> measure to keep cgroups around if they are bound to a resource which is
> sticky itself. I am not sure how many other resources like BPF (aka
> module like) we already do charge for memcg but considering the
> potential memory consumption just reparenting will not help in general
> case I am afraid.

Well, then we have to make these objects a first-class citizens in cgroup API,
like processes. E.g. introduce cgroup.bpf.maps, cgroup.mounts.tmpfs etc.
I easily can see some value here, but it's a big API change.

With the current approach when a bpf map pins a memory cgroup of the creator
process (which I think is completely transparent for most bpf users), I don't
think preventing the deletion of a such cgroup is possible. It will break too
many things.

But honestly I don't see why userspace can't handle it. If there is a cgroup which
contains shared bpf maps, why would it delete it? It's a weird use case, I don't
think we have to optimize for it. Also, we do a ton of optimizations for live
cgroups (e.g. css refcounting being percpu) which are not working for a deleted
cgroup. So noone really should expect any properties from dying cgroups.

Thanks!