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

[Shakeel Butt <shakeelb@google.com>]

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

>
> [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*

> 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

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.

>
> 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
>