diff --git a/drivers/md/persistent-data/dm-space-map-common.c b/drivers/md/persistent-data/dm-space-map-common.c
index 6058569..466a60b 100644
--- a/drivers/md/persistent-data/dm-space-map-common.c
+++ b/drivers/md/persistent-data/dm-space-map-common.c
@@ -381,7 +381,7 @@
 }
 
 static int sm_ll_mutate(struct ll_disk *ll, dm_block_t b,
-			uint32_t (*mutator)(void *context, uint32_t old),
+			int (*mutator)(void *context, uint32_t old, uint32_t *new),
 			void *context, enum allocation_event *ev)
 {
 	int r;
@@ -410,11 +410,17 @@
 
 	if (old > 2) {
 		r = sm_ll_lookup_big_ref_count(ll, b, &old);
-		if (r < 0)
+		if (r < 0) {
+			dm_tm_unlock(ll->tm, nb);
 			return r;
+		}
 	}
 
-	ref_count = mutator(context, old);
+	r = mutator(context, old, &ref_count);
+	if (r) {
+		dm_tm_unlock(ll->tm, nb);
+		return r;
+	}
 
 	if (ref_count <= 2) {
 		sm_set_bitmap(bm_le, bit, ref_count);
@@ -465,9 +471,10 @@
 	return ll->save_ie(ll, index, &ie_disk);
 }
 
-static uint32_t set_ref_count(void *context, uint32_t old)
+static int set_ref_count(void *context, uint32_t old, uint32_t *new)
 {
-	return *((uint32_t *) context);
+	*new = *((uint32_t *) context);
+	return 0;
 }
 
 int sm_ll_insert(struct ll_disk *ll, dm_block_t b,
@@ -476,9 +483,10 @@
 	return sm_ll_mutate(ll, b, set_ref_count, &ref_count, ev);
 }
 
-static uint32_t inc_ref_count(void *context, uint32_t old)
+static int inc_ref_count(void *context, uint32_t old, uint32_t *new)
 {
-	return old + 1;
+	*new = old + 1;
+	return 0;
 }
 
 int sm_ll_inc(struct ll_disk *ll, dm_block_t b, enum allocation_event *ev)
@@ -486,9 +494,15 @@
 	return sm_ll_mutate(ll, b, inc_ref_count, NULL, ev);
 }
 
-static uint32_t dec_ref_count(void *context, uint32_t old)
+static int dec_ref_count(void *context, uint32_t old, uint32_t *new)
 {
-	return old - 1;
+	if (!old) {
+		DMERR_LIMIT("unable to decrement a reference count below 0");
+		return -EINVAL;
+	}
+
+	*new = old - 1;
+	return 0;
 }
 
 int sm_ll_dec(struct ll_disk *ll, dm_block_t b, enum allocation_event *ev)