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

[Michal Hocko <mhocko@kernel.org>]

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.

-- 
Michal Hocko
SUSE Labs