oom: do not kill tasks with oom_score_adj OOM_SCORE_ADJ_MIN

Commit c9f01245 ("oom: remove oom_disable_count") has removed the
oom_disable_count counter which has been used for early break out from
oom_badness so we could never select a task with oom_score_adj set to
OOM_SCORE_ADJ_MIN (oom disabled).

Now that the counter is gone we are always going through heuristics
calculation and we always return a non zero positive value.  This means
that we can end up killing a task with OOM disabled because it is
indistinguishable from regular tasks with 1% resp.  CAP_SYS_ADMIN tasks
with 3% usage of memory or tasks with oom_score_adj set but OOM enabled.

Let's break out early if the task should have OOM disabled.
Signed-off-by: Michal Hocko <mhocko@suse.cz>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Ying Han <yinghan@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/oom_kill.c

@@ -185,6 +185,11 @@ unsigned int oom_badness(struct task_struct *p, struct mem_cgroup *mem,
 	if (!p)
 		return 0;
 
+	if (p->signal->oom_score_adj == OOM_SCORE_ADJ_MIN) {
+		task_unlock(p);
+		return 0;
+	}
+
 	/*
 	 * The memory controller may have a limit of 0 bytes, so avoid a divide
 	 * by zero, if necessary.