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

[Michal Hocko <mhocko@kernel.org>]

On Mon 01-07-19 22:04:22, Tetsuo Handa wrote:
> On 2019/07/01 20:17, Michal Hocko wrote:
> > On Sat 29-06-19 20:24:34, Tetsuo Handa wrote:
> >> Since mpol_put_task_policy() in do_exit() sets mempolicy = NULL,
> >> mempolicy_nodemask_intersects() considers exited threads (e.g. a process
> >> with dying leader and live threads) as eligible. But it is possible that
> >> all of live threads are still ineligible.
> >>
> >> Since has_intersects_mems_allowed() returns true as soon as one of threads
> >> is considered eligible, mempolicy_nodemask_intersects() needs to consider
> >> exited threads as ineligible. Since exit_mm() in do_exit() sets mm = NULL
> >> before mpol_put_task_policy() sets mempolicy = NULL, we can exclude exited
> >> threads by checking whether mm is NULL.
> > 
> > Ok, this makes sense. For this change
> > Acked-by: Michal Hocko <mhocko@suse.com>
> > 
> 
> 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?

> If
> we fail to allow the process to reach MMF_OOM_SKIP test, the process will be
> ignored by the OOM killer as soon as all threads pass mm = NULL at exit_mm(), for
> has_intersects_mems_allowed() returns false unless MPOL_{BIND,INTERLEAVE} is used.
> 
> Well, the problem is that exited threads prematurely set mempolicy = NULL.
> Since bitmap memory for cpuset_mems_allowed_intersects() path is freed when
> __put_task_struct() is called, mempolicy memory for mempolicy_nodemask_intersects()
> path should be freed as well when __put_task_struct() is called?

I am sorry but I have hard time understanding what is the actual user
visible problem here.
-- 
Michal Hocko
SUSE Labs