Re: [PATCH resend] mm, memcg: fix inconsistent oom event behavior

[Michal Hocko <mhocko@kernel.org>]

On Mon 04-05-20 16:03:45, Andrew Morton wrote:
> On Sat,  2 May 2020 10:10:55 -0400 Yafang Shao <laoar.shao@gmail.com> 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