oom: PF_EXITING check should take mm into account

select_bad_process() checks PF_EXITING to detect the task which is going
to release its memory, but the logic is very wrong.

	- a single process P with the dead group leader disables
	  select_bad_process() completely, it will always return
	  ERR_PTR() while P can live forever

	- if the PF_EXITING task has already released its ->mm
	  it doesn't make sense to expect it is goiing to free
	  more memory (except task_struct/etc)

Change the code to ignore the PF_EXITING tasks without ->mm.

Signed-off-by: Oleg Nesterov <[email protected]>
Signed-off-by: David Rientjes <[email protected]>
Cc: Balbir Singh <[email protected]>
Acked-by: KOSAKI Motohiro <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: oleg-nesterov

mm/oom_kill.c

@@ -287,7 +287,7 @@ static struct task_struct *select_bad_process(unsigned long *ppoints,
 		 * the process of exiting and releasing its resources.
 		 * Otherwise we could get an easy OOM deadlock.
 		 */
-		if (p->flags & PF_EXITING) {
+		if ((p->flags & PF_EXITING) && p->mm) {
 			if (p != current)
 				return ERR_PTR(-1UL);