On Mon 04-05-20 16:03:45, Andrew Morton wrote:
> On Sat, 2 May 2020 10:10:55 -0400 Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
>
> > A recent commit 9852ae3fe529 ("mm, memcg: consider subtrees in
> > memory.events") changes the behavior of memcg events, which will
> > consider subtrees in memory.events. But oom_kill event is a special one
> > as it is used in both cgroup1 and cgroup2. In cgroup1, it is displayed
> > in memory.oom_control. The file memory.oom_control is in both root memcg
> > and non root memcg, that is different with memory.event as it only in
> > non-root memcg. That commit is okay for cgroup2, but it is not okay for
> > cgroup1 as it will cause inconsistent behavior between root memcg and
> > non-root memcg.
> >
> > Here's an example on why this behavior is inconsistent in cgroup1.
> > root memcg
> > /
> > memcg foo
> > /
> > memcg bar
> >
> > Suppose there's an oom_kill in memcg bar, then the oon_kill will be
> >
> > root memcg : memory.oom_control(oom_kill) 0
> > /
> > memcg foo : memory.oom_control(oom_kill) 1
> > /
> > memcg bar : memory.oom_control(oom_kill) 1
> >
> > For the non-root memcg, its memory.oom_control(oom_kill) includes its
> > descendants' oom_kill, but for root memcg, it doesn't include its
> > descendants' oom_kill. That means, memory.oom_control(oom_kill) has
> > different meanings in different memcgs. That is inconsistent. Then the user
> > has to know whether the memcg is root or not.
> >
> > If we can't fully support it in cgroup1, for example by adding
> > memory.events.local into cgroup1 as well, then let's don't touch
> > its original behavior.
> >
> > Fixes: 9852ae3fe529 ("mm, memcg: consider subtrees in memory.events")
>
> Nearly a year ago. Should we backport this into earlier kernels?
It is a trivial change so I do not see problem marking it for stable.
>
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -783,6 +783,8 @@ static inline void memcg_memory_event(struct mem_cgroup *memcg,
> > atomic_long_inc(&memcg->memory_events[event]);
> > cgroup_file_notify(&memcg->events_file);
> >
> > + if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
> > + break;
> > if (cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_LOCAL_EVENTS)
> > break;
> > } while ((memcg = parent_mem_cgroup(memcg)) &&
--
Michal Hocko
SUSE Labs
