Re: [PATCH] mm, oom: fix oom_unkillable_task for memcg OOMs

[Shakeel Butt <shakeelb@google.com>]

On Mon, Jun 17, 2019 at 9:17 AM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Mon 17-06-19 08:59:54, Shakeel Butt wrote:
> > Currently oom_unkillable_task() checks mems_allowed even for memcg OOMs
> > which does not make sense as memcg OOMs can not be triggered due to
> > numa constraints. Fixing that.
> >
> > Also if memcg is given, oom_unkillable_task() will check the task's
> > memcg membership as well to detect oom killability. However all the
> > memcg related code paths leading to oom_unkillable_task(), other than
> > dump_tasks(), come through mem_cgroup_scan_tasks() which traverses
> > tasks through memcgs. Once dump_tasks() is converted to use
> > mem_cgroup_scan_tasks(), there is no need to do memcg membership check
> > in oom_unkillable_task().
>
> I think this patch just does too much in one go. Could you split out
> the dump_tasks part and the oom_unkillable_task parts into two patches
> please? It should be slightly easier to review.
>

Yes, will do in v2.

> [...]
> > +static bool oom_unkillable_task(struct task_struct *p, struct oom_control *oc)
> >  {
> >       if (is_global_init(p))
> >               return true;
> >       if (p->flags & PF_KTHREAD)
> >               return true;
> > +     if (!oc)
> > +             return false;
>
> Bah, this is just too ugly. AFAICS this is only because oom_score still
> uses oom_unkillable_task which is kinda dubious, no? While you are
> touching this code, can we remove this part as well? I would be really
> surprised if any code really depends on ineligible tasks reporting 0
> oom_score.

I think it is safer to just localize the is_global_init() and
PF_KTHREAD checks in oom_badness() instead of invoking
oom_unkillable_task(). Also I think cpuset_mems_allowed_intersects()
check from /proc/[pid]/oom_score is unintentional.

Shakeel