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

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On Wed, Jul 06, 2022 at 10:46:48AM +0800, Yafang Shao wrote:
> On Wed, Jul 6, 2022 at 4:49 AM Roman Gushchin <roman.gushchin@xxxxxxxxx> 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@xxxxxxxxx> 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.

What do you mean? Can you please elaborate on it?

Thanks!



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