Re: [PATCH 1/1] mm, oom_adj: don't loop through tasks in __set_oom_adj when not necessary

[Oleg Nesterov <oleg@redhat.com>]

On 08/20, Eric W. Biederman wrote:
>
> That said if we are going for a small change why not:
>
> 	/*
> 	 * Make sure we will check other processes sharing the mm if this is
> 	 * not vfrok which wants its own oom_score_adj.
> 	 * pin the mm so it doesn't go away and get reused after task_unlock
> 	 */
> 	if (!task->vfork_done) {
> 		struct task_struct *p = find_lock_task_mm(task);
>
> 		if (p) {
> -			if (atomic_read(&p->mm->mm_users) > 1) {
> +			if (atomic_read(&p->mm->mm_users) > p->signal->nr_threads) {

In theory this needs a barrier to avoid the race with do_exit(). And I'd
suggest to use signal->live, I think signal->nr_threads should die...
Something like

	bool probably_has_other_mm_users(tsk)
	{
		return	atomic_read_acquire(&tsk->mm->mm_users) >
			atomic_read(&tsk->signal->live);
	}

The barrier implied by _acquire ensures that if we race with the exiting
task and see the result of exit_mm()->mmput(mm), then we must also see
the result of atomic_dec_and_test(signal->live).

Either way, if we want to fix the race with clone(CLONE_VM) we need other
changes.

Oleg.