Re: [PATCH] mm: memcontrol: do not miss MEMCG_MAX events for enforced allocations

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

On Mon, Jul 04, 2022 at 05:07:30PM +0200, Michal Hocko wrote:
> On Sat 02-07-22 08:39:14, Roman Gushchin wrote:
> > On Fri, Jul 01, 2022 at 10:50:40PM -0700, Shakeel Butt wrote:
> > > On Fri, Jul 1, 2022 at 8:35 PM Roman Gushchin <roman.gushchin@linux.dev> wrote:
> > > >
> > > > Yafang Shao reported an issue related to the accounting of bpf
> > > > memory: if a bpf map is charged indirectly for memory consumed
> > > > from an interrupt context and allocations are enforced, MEMCG_MAX
> > > > events are not raised.
> > > >
> > > > It's not/less of an issue in a generic case because consequent
> > > > allocations from a process context will trigger the reclaim and
> > > > MEMCG_MAX events. However a bpf map can belong to a dying/abandoned
> > > > memory cgroup, so it might never happen.
> > > 
> > > The patch looks good but the above sentence is confusing. What might
> > > never happen? Reclaim or MAX event on dying memcg?
> > 
> > Direct reclaim and MAX events. I agree it might be not clear without
> > looking into the code. How about something like this?
> > 
> > "It's not/less of an issue in a generic case because consequent
> > allocations from a process context will trigger the direct reclaim
> > and MEMCG_MAX events will be raised. However a bpf map can belong
> > to a dying/abandoned memory cgroup, so there will be no allocations
> > from a process context and no MEMCG_MAX events will be triggered."
> 
> Could you expand little bit more on the situation? Can those charges to
> offline memcg happen indefinetely?

Yes.

> How can it ever go away then?

Bpf map should be deleted by a user first.

> Also is this something that we actually want to encourage?

Not really. We can implement reparenting (probably objcg-based), I think it's
a good idea in general. I can take a look, but can't promise it will be fast.

In thory we can't forbid deleting cgroups with associated bpf maps, but I don't
thinks it's a good idea.

> In other words shouldn't those remote charges be redirected when the
> target memcg is offline?

Reparenting is the best answer I have.

Thanks!