diff --git a/drivers/md/persistent-data/dm-space-map-disk.c b/drivers/md/persistent-data/dm-space-map-disk.c
index f6d29e6..e735a6d 100644
--- a/drivers/md/persistent-data/dm-space-map-disk.c
+++ b/drivers/md/persistent-data/dm-space-map-disk.c
@@ -248,7 +248,8 @@
 	.new_block = sm_disk_new_block,
 	.commit = sm_disk_commit,
 	.root_size = sm_disk_root_size,
-	.copy_root = sm_disk_copy_root
+	.copy_root = sm_disk_copy_root,
+	.register_threshold_callback = NULL
 };
 
 struct dm_space_map *dm_sm_disk_create(struct dm_transaction_manager *tm,
diff --git a/drivers/md/persistent-data/dm-space-map-metadata.c b/drivers/md/persistent-data/dm-space-map-metadata.c
index 51ca9ed..883b465 100644
--- a/drivers/md/persistent-data/dm-space-map-metadata.c
+++ b/drivers/md/persistent-data/dm-space-map-metadata.c
@@ -391,7 +391,8 @@
 	.new_block = sm_metadata_new_block,
 	.commit = sm_metadata_commit,
 	.root_size = sm_metadata_root_size,
-	.copy_root = sm_metadata_copy_root
+	.copy_root = sm_metadata_copy_root,
+	.register_threshold_callback = NULL
 };
 
 /*----------------------------------------------------------------*/
@@ -513,7 +514,8 @@
 	.new_block = sm_bootstrap_new_block,
 	.commit = sm_bootstrap_commit,
 	.root_size = sm_bootstrap_root_size,
-	.copy_root = sm_bootstrap_copy_root
+	.copy_root = sm_bootstrap_copy_root,
+	.register_threshold_callback = NULL
 };
 
 /*----------------------------------------------------------------*/
diff --git a/drivers/md/persistent-data/dm-space-map.h b/drivers/md/persistent-data/dm-space-map.h
index 1cbfc6b..3e6d115 100644
--- a/drivers/md/persistent-data/dm-space-map.h
+++ b/drivers/md/persistent-data/dm-space-map.h
@@ -9,6 +9,8 @@
 
 #include "dm-block-manager.h"
 
+typedef void (*dm_sm_threshold_fn)(void *context);
+
 /*
  * struct dm_space_map keeps a record of how many times each block in a device
  * is referenced.  It needs to be fixed on disk as part of the transaction.
@@ -59,6 +61,15 @@
 	 */
 	int (*root_size)(struct dm_space_map *sm, size_t *result);
 	int (*copy_root)(struct dm_space_map *sm, void *copy_to_here_le, size_t len);
+
+	/*
+	 * You can register one threshold callback which is edge-triggered
+	 * when the free space in the space map drops below the threshold.
+	 */
+	int (*register_threshold_callback)(struct dm_space_map *sm,
+					   dm_block_t threshold,
+					   dm_sm_threshold_fn fn,
+					   void *context);
 };
 
 /*----------------------------------------------------------------*/
@@ -131,4 +142,16 @@
 	return sm->copy_root(sm, copy_to_here_le, len);
 }
 
+static inline int dm_sm_register_threshold_callback(struct dm_space_map *sm,
+						    dm_block_t threshold,
+						    dm_sm_threshold_fn fn,
+						    void *context)
+{
+	if (sm->register_threshold_callback)
+		return sm->register_threshold_callback(sm, threshold, fn, context);
+
+	return -EINVAL;
+}
+
+
 #endif	/* _LINUX_DM_SPACE_MAP_H */