Re: [PATCH] mm: mempolicy: don't select exited threads as OOM victims

[Michal Hocko <mhocko@kernel.org>]

On Mon 01-07-19 22:56:12, Tetsuo Handa wrote:
> On 2019/07/01 22:48, Michal Hocko wrote:
> > On Mon 01-07-19 22:38:58, Tetsuo Handa wrote:
> >> On 2019/07/01 22:17, Michal Hocko wrote:
> >>> On Mon 01-07-19 22:04:22, Tetsuo Handa wrote:
> >>>> But I realized that this patch was too optimistic. We need to wait for mm-less
> >>>> threads until MMF_OOM_SKIP is set if the process was already an OOM victim.
> >>>
> >>> If the process is an oom victim then _all_ threads are so as well
> >>> because that is the address space property. And we already do check that
> >>> before reaching oom_badness IIRC. So what is the actual problem you are
> >>> trying to solve here?
> >>
> >> I'm talking about behavioral change after tsk became an OOM victim.
> >>
> >> If tsk->signal->oom_mm != NULL, we have to wait for MMF_OOM_SKIP even if
> >> tsk->mm == NULL. Otherwise, the OOM killer selects next OOM victim as soon as
> >> oom_unkillable_task() returned true because has_intersects_mems_allowed() returned
> >> false because mempolicy_nodemask_intersects() returned false because all thread's
> >> mm became NULL (despite tsk->signal->oom_mm != NULL).
> > 
> > OK, I finally got your point. It was not clear that you are referring to
> > the code _after_ the patch you are proposing. You are indeed right that
> > this would have a side effect that an additional victim could be
> > selected even though the current process hasn't terminated yet. Sigh,
> > another example how the whole thing is subtle so I retract my Ack and
> > request a real life example of where this matters before we think about
> > a proper fix and make the code even more complex.
> > 
> 
> Instead of checking for mm != NULL, can we move mpol_put_task_policy() from
> do_exit() to __put_task_struct() ? That change will (if it is safe to do)
> prevent exited threads from setting mempolicy = NULL (and confusing
> mempolicy_nodemask_intersects() due to mempolicy == NULL).

I am sorry but I would have to study it much more and I am not convinced
the time spent on it would be well spent.

-- 
Michal Hocko
SUSE Labs