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

[Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>]

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).

static int oom_evaluate_task(struct task_struct *task, void *arg)
{
  if (oom_unkillable_task(task, NULL, oc->nodemask))
    goto next;
  if (!is_sysrq_oom(oc) && tsk_is_oom_victim(task)) {
    if (test_bit(MMF_OOM_SKIP, &task->signal->oom_mm->flags))
      goto next;
    goto abort;
  }
}