dm cache policy smq: allow demotions to happen even during continuous IO

dm-cache's smq policy tries hard to do it's work during the idle periods
when there is no IO.  But if there are no idle periods (eg, a long fio
run) we still need to allow some demotions and promotions to occur.

To achieve this, pass @idle=true to queue_promotion()'s
free_target_met() call so that free_target_met() doesn't short-circuit
the possibility of demotion simply because it isn't an idle period.

Fixes: b29d4986d0 ("dm cache: significant rework to leverage dm-bio-prison-v2")
Reported-by: John Harrigan <jharriga@redhat.com>
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

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

@@ -1214,7 +1214,11 @@ static void queue_promotion(struct smq_policy *mq, dm_oblock_t oblock,
 		return;
 
 	if (allocator_empty(&mq->cache_alloc)) {
-		if (!free_target_met(mq, false))
+		/*
+		 * We always claim to be 'idle' to ensure some demotions happen
+		 * with continuous loads.
+		 */
+		if (!free_target_met(mq, true))
 			queue_demotion(mq);
 		return;
 	}