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

[Michal Hocko <mhocko@suse.com>]

On Wed 06-07-22 10:40:48, Yafang Shao wrote:
> On Wed, Jul 6, 2022 at 4:52 AM Roman Gushchin <roman.gushchin@linux.dev> wrote:
> >
> > On Mon, Jul 04, 2022 at 05:30:25PM +0200, Michal Hocko wrote:
> > > 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?
> >
> > Idk, the whole problem was reported by Yafang, so he probably has a better
> > answer. But in general events are recursive and the cgroup doesn't have
> > to be dying, it can be simple abandoned.
> >
> 
> Regarding the pinned bpf programs, it can run without a user agent.
> That means the cgroup may not be dead, but just not populated.
> (But in our case, the cgroup will be deleted after the user agent exits.)

OK, that makes sense.
-- 
Michal Hocko
SUSE Labs