Re: [PATCH 1/6] mm, oom: do not loop over all tasks if there are no external tasks sharing mm

[Tetsuo Handa]

Michal Hocko wrote:
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 5bb2f7698ad7..0e33e912f7e4 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -820,6 +820,13 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
>  	task_unlock(victim);
>  
>  	/*
> +	 * skip expensive iterations over all tasks if we know that there
> +	 * are no users outside of threads in the same thread group
> +	 */
> +	if (atomic_read(&mm->mm_users) <= get_nr_threads(victim))
> +		goto oom_reap;

Is this really safe? Isn't it possible that victim thread's thread group has
more than atomic_read(&mm->mm_users) threads which are past exit_mm() and blocked
at exit_task_work() which are before __exit_signal() from release_task() from
exit_notify()?

> +
> +	/*
>  	 * Kill all user processes sharing victim->mm in other thread groups, if
>  	 * any.  They don't get access to memory reserves, though, to avoid
>  	 * depletion of all memory.  This prevents mm->mmap_sem livelock when an