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

[Yafang Shao <laoar.shao@gmail.com>]

On Wed, Jul 6, 2022 at 4:49 AM Roman Gushchin <roman.gushchin@linux.dev> wrote:
>
> 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.
>

It can't apply to pinned bpf maps, because the user expects the bpf
maps to continue working after the user agent exits.

> > 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.
>

Agreed. It is not 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.
>

At the cost of increasing the complexity of deployment, that may not
be a good idea neither.

-- 
Regards
Yafang