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

Oleg Nesterov
2020-08-21T11:16:58Z

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.