oom: oom_kill_process() needs to check that p is unkillable

When oom_kill_allocating_task is enabled, an argument task of
oom_kill_process is not selected by select_bad_process(), It's just
out_of_memory() caller task.  It mean the task can be unkillable.  check
it first.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Cc: David Rientjes <rientjes@google.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/oom_kill.c

@@ -687,7 +687,8 @@ void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
 	check_panic_on_oom(constraint, gfp_mask, order);
 
 	read_lock(&tasklist_lock);
-	if (sysctl_oom_kill_allocating_task) {
+	if (sysctl_oom_kill_allocating_task &&
+	    !oom_unkillable_task(current, NULL, nodemask)) {
 		/*
 		 * oom_kill_process() needs tasklist_lock held.  If it returns
 		 * non-zero, current could not be killed so we must fallback to