On Fri 27-05-16 00:25:23, Tetsuo Handa wrote:

Michal Hocko wrote:

quoted

On Thu 26-05-16 23:30:06, Tetsuo Handa wrote:

quoted

Michal Hocko wrote:

quoted

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()?

You are right. The race window between exit_mm and __exit_signal is
really large. I thought about == check instead but that wouldn't work
for the same reason, dang, it looked so promissing.

Scratch this patch then.

I think that remembering whether this mm might be shared between
multiple thread groups at clone() time (i.e. whether
clone(CLONE_VM without CLONE_SIGHAND) was ever requested on this mm)
is safe (given that that thread already got SIGKILL or is exiting).

I was already playing with that idea but I didn't want to add anything
to the fork path which is really hot. This patch is not really needed
for the rest. It just felt like a nice optimization. I do not think it
is worth deeper changes in the fast paths.

By the way, in oom_kill_process(), how (p->flags & PF_KTHREAD) case can
become true when process_shares_mm() is true?

not sure I understand. But the PF_KTHREAD check is there to catch
use_mm() usage by kernel threads.

Even if it can become true,
why can't we reap that mm? Is (p->flags & PF_KTHREAD) case only for
not to send SIGKILL rather than not to reap that mm?

If we reaped the mm then the kernel thread could blow up when accessing
a memory.

-- 
Michal Hocko
SUSE Labs

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