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

[Michal Hocko <mhocko@suse.com>]

On Mon 04-07-22 17:07:32, 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? How can it ever go away then? Also is
> this something that we actually want to encourage?

One more question. Mostly out of curiosity. How is userspace actually
acting on those events? Are watchers still active on those dead memcgs?
-- 
Michal Hocko
SUSE Labs