Re: [PATCH 2/2] mm, memcg: don't try to kill a process if memcg is not populated

[Yafang Shao <laoar.shao@gmail.com>]

On Mon, May 4, 2020 at 7:00 AM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Sat, May 2, 2020 at 7:35 AM Yafang Shao <laoar.shao@gmail.com> wrote:
> >
> > Recently Shakeel reported a issue which also confused me serveral months
> > earlier. Bellow is his report -
> >
> > Lowering memory.max can trigger an oom-kill if the reclaim does not
> > succeed. However if oom-killer does not find a process for killing, it
> > dumps a lot of warnings.
> >
> > Deleting a memcg does not reclaim memory from it and the memory can
> > linger till there is a memory pressure. One normal way to proactively
> > reclaim such memory is to set memory.max to 0 just before deleting the
> > memcg. However if some of the memcg's memory is pinned by others, this
> > operation can trigger an oom-kill without any process and thus can log a
> > lot un-needed warnings. So, ignore all such warnings from memory.max.
>
> lot *of* un-needed
>

Thanks.

> >
> > [shakeelb@google.com: commit log above]
> >
> > A better way to avoid this issue is to avoid trying to kill a process if
> > memcg is not populated.
> > Note that OOM is different with OOM kill.
>
> different *from*
>

Thanks

> > OOM is a status that the
> > system or memcg is out of memory, while OOM kill is a result that a
> > process inside this memcg is killed when this memcg is in OOM status.
> > That is the same reason why there're both MEMCG_OOM event and
> > MEMCG_OOM_KILL event. If we have already known that there's nothing to
> > kill, i.e. the memcg is not populated, then we don't need to have a try.
>
> need to try
>

Thanks

> I think adding the discussion of memory.high is also useful in the
> commit message. Basically why setting memory.max to 0 is better than
> setting memory.high to 0 before deletion. The reason is remote
> charging.
>

Sure, will add it.

> >
> > Cc: Shakeel Butt <shakeelb@google.com>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Cc: Michal Hocko <mhocko@kernel.org>
> > Cc: Roman Gushchin <guro@fb.com>
> > Cc: Greg Thelen <gthelen@google.com>
> > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
>
> Reviewed-by: Shakeel Butt <shakeelb@google.com>
>
> > ---
> >  mm/memcontrol.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 985edce98491..29afe3df9d98 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -6102,6 +6102,10 @@ static ssize_t memory_max_write(struct kernfs_open_file *of,
> >                 }
> >
> >                 memcg_memory_event(memcg, MEMCG_OOM);
> > +
> > +               if (!cgroup_is_populated(memcg->css.cgroup))
> > +                       break;
> > +
> >                 if (!mem_cgroup_oom_kill(memcg, GFP_KERNEL, 0))
> >                         break;
> >         }
> > --
> > 2.18.2
> >



-- 
Thanks
Yafang