dm cache background tracker: limit amount of background work that may be issued at once

On large systems the cache policy can be over enthusiastic and queue far
too much dirty data to be written back.  This consumes memory.
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-cache-background-tracker.c

@@ -161,8 +161,17 @@ EXPORT_SYMBOL_GPL(btracker_nr_demotions_queued);
 
 static bool max_work_reached(struct background_tracker *b)
 {
-	// FIXME: finish
-	return false;
+	return atomic_read(&b->pending_promotes) +
+		atomic_read(&b->pending_writebacks) +
+		atomic_read(&b->pending_demotes) >= b->max_work;
+}
+
+struct bt_work *alloc_work(struct background_tracker *b)
+{
+	if (max_work_reached(b))
+		return NULL;
+
+	return kmem_cache_alloc(b->work_cache, GFP_NOWAIT);
 }
 
 int btracker_queue(struct background_tracker *b,
@@ -174,10 +183,7 @@ int btracker_queue(struct background_tracker *b,
 	if (pwork)
 		*pwork = NULL;
 
-	if (max_work_reached(b))
-		return -ENOMEM;
-
-	w = kmem_cache_alloc(b->work_cache, GFP_NOWAIT);
+	w = alloc_work(b);
 	if (!w)
 		return -ENOMEM;