oom, oom_reaper: allow to reap mm shared by the kthreads

oom reaper was skipped for an mm which is shared with the kernel thread
(aka use_mm()).  The primary concern was that such a kthread might want
to read from the userspace memory and see zero page as a result of the
oom reaper action.  This is no longer a problem after "mm: make sure
that kthreads will not refault oom reaped memory" because any attempt to
fault in when the MMF_UNSTABLE is set will result in SIGBUS and so the
target user should see an error.  This means that we can finally allow
oom reaper also to tasks which share their mm with kthreads.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Michal Hocko <[email protected]>
Cc: Tetsuo Handa <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/oom_kill.c

@@ -902,20 +902,20 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
 			continue;
 		if (same_thread_group(p, victim))
 			continue;
-		if (unlikely(p->flags & PF_KTHREAD) || is_global_init(p)) {
-			/*
-			 * We cannot use oom_reaper for the mm shared by this
-			 * process because it wouldn't get killed and so the
-			 * memory might be still used. Hide the mm from the oom
-			 * killer to guarantee OOM forward progress.
-			 */
+		if (is_global_init(p)) {
 			can_oom_reap = false;
 			set_bit(MMF_OOM_SKIP, &mm->flags);
 			pr_info("oom killer %d (%s) has mm pinned by %d (%s)\n",
 					task_pid_nr(victim), victim->comm,
 					task_pid_nr(p), p->comm);
 			continue;
 		}
+		/*
+		 * No use_mm() user needs to read from the userspace so we are
+		 * ok to reap it.
+		 */
+		if (unlikely(p->flags & PF_KTHREAD))
+			continue;
 		do_send_sig_info(SIGKILL, SEND_SIG_FORCED, p, true);
 	}
 	rcu_read_unlock();