oom: avoid unnecessary mm locking and scanning for OOM_DISABLE

This moves the check for OOM_DISABLE to the badness heuristic so it is
only necessary to hold task_lock() once.  If the mm is OOM_DISABLE, the
score is 0, which is also correctly exported via /proc/pid/oom_score.
This requires that tasks with badness scores of 0 are prohibited from
being oom killed, which makes sense since they would not allow for future
memory freeing anyway.

Since the oom_adj value is a characteristic of an mm and not a task, it is
no longer necessary to check the oom_adj value for threads sharing the
same memory (except when simply issuing SIGKILLs for threads in other
thread groups).

Cc: Nick Piggin <npiggin@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/oom_kill.c

@@ -67,6 +67,10 @@ unsigned long badness(struct task_struct *p, unsigned long uptime)
 		return 0;
 	}
 	oom_adj = mm->oom_adj;
+	if (oom_adj == OOM_DISABLE) {
+		task_unlock(p);
+		return 0;
+	}
 
 	/*
 	 * The memory size of the process is the basis for the badness.
@@ -253,15 +257,8 @@ static struct task_struct *select_bad_process(unsigned long *ppoints,
 			*ppoints = ULONG_MAX;
 		}
 
-		task_lock(p);
-		if (p->mm && p->mm->oom_adj == OOM_DISABLE) {
-			task_unlock(p);
-			continue;
-		}
-		task_unlock(p);
-
 		points = badness(p, uptime.tv_sec);
-		if (points > *ppoints || !chosen) {
+		if (points > *ppoints) {
 			chosen = p;
 			*ppoints = points;
 		}
@@ -354,32 +351,13 @@ static int oom_kill_task(struct task_struct *p)
 	struct mm_struct *mm;
 	struct task_struct *g, *q;
 
+	task_lock(p);
 	mm = p->mm;
-
-	/* WARNING: mm may not be dereferenced since we did not obtain its
-	 * value from get_task_mm(p).  This is OK since all we need to do is
-	 * compare mm to q->mm below.
-	 *
-	 * Furthermore, even if mm contains a non-NULL value, p->mm may
-	 * change to NULL at any time since we do not hold task_lock(p).
-	 * However, this is of no concern to us.
-	 */
-
-	if (mm == NULL)
-		return 1;
-
-	/*
-	 * Don't kill the process if any threads are set to OOM_DISABLE
-	 */
-	do_each_thread(g, q) {
-		task_lock(q);
-		if (q->mm == mm && q->mm && q->mm->oom_adj == OOM_DISABLE) {
-			task_unlock(q);
+	if (!mm || mm->oom_adj == OOM_DISABLE) {
+		task_unlock(p);
 		return 1;
 	}
-		task_unlock(q);
-	} while_each_thread(g, q);
-
+	task_unlock(p);
 	__oom_kill_task(p, 1);
 
 	/*