dm cache policy smq: only demote entries in bottom half of the clean multiqueue

Heavy IO load may mean there are very few clean blocks in the cache, and
we risk demoting entries that get hit a lot.
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-cache-policy-smq.c

@@ -1190,7 +1190,7 @@ static void queue_demotion(struct smq_policy *mq)
 	if (unlikely(WARN_ON_ONCE(!mq->migrations_allowed)))
 		return;
 
-	e = q_peek(&mq->clean, mq->clean.nr_levels, true);
+	e = q_peek(&mq->clean, mq->clean.nr_levels / 2, true);
 	if (!e) {
 		if (!clean_target_met(mq, false))
 			queue_writeback(mq);