On Fri 27-05-16 09:15:07, Michal Hocko wrote:
> On Fri 27-05-16 08:45:10, Michal Hocko wrote:
> [...]
> > It is still an operation which is not needed for 99% of situations. So
> > if we do not need it for correctness then I do not think this is worth
> > bothering.
>
> Since you have pointed out exit_mm vs. __exit_signal race yesterday I
> was thinking how to make the check reliable. Even
> atomic_read(mm->mm_users) > get_nr_threads() is not reliable and we can
> miss other tasks just because the current thread group is mostly past
> exit_mm. So far I couldn't find a way to tweak this around though.
Just for the record I was playing with the following yesterday but I
couldn't convince myself that this is safe and reasonable in the first
place (I do not like it to be honest).
---
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 1685890d424e..db027eca8be5 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -123,6 +123,35 @@ struct task_struct *find_lock_task_mm(struct task_struct
*p)
return t;
}
+bool task_has_external_users(struct task_struct *p)
+{
+ struct mm_struct *mm = NULL;
+ struct task_struct *t;
+ int active_threads = 0;
+ bool ret = true; /* be pessimistic */
+
+ rcu_read_lock();
+ for_each_thread(p, t) {
+ task_lock(t);
+ if (likely(t->mm)) {
+ active_threads++;
+ if (!mm) {
+ mm = t->mm;
+ atomic_inc(&mm->mm_count);
+ }
+ }
+ task_unlock(t);
+ }
+ rcu_read_unlock();
+
+ if (mm) {
+ if (atomic_read(&mm->mm_users) <= active_threads)
+ ret = false;
+ mmdrop(mm);
+ }
+ return ret;
+}
+
/*
* order == -1 means the oom kill is required by sysrq, otherwise only
* for display purposes.
--
Michal Hocko
SUSE Labs
