dm table: add singleton feature

Introduce the concept of a singleton table which contains exactly one target.

If a target type sets the DM_TARGET_SINGLETON feature bit device-mapper
will ensure that any table that includes that target contains no others.

The thin provisioning pool target uses this.

Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 10 insertions, 4 deletions

diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
index 622678ccb5e..294e78a7fcc 100644
--- a/include/linux/device-mapper.h
+++ b/include/linux/device-mapper.h
@@ -128,10 +128,6 @@ void dm_put_device(struct dm_target *ti, struct dm_dev *d);
  * Information about a target type
  */
 
-/*
- * Target features
- */
-
 struct target_type {
 	uint64_t features;
 	const char *name;
@@ -160,6 +156,16 @@ struct target_type {
 	struct list_head list;
 };
 
+/*
+ * Target features
+ */
+
+/*
+ * Any table that contains an instance of this target must have only one.
+ */
+#define DM_TARGET_SINGLETON		0x00000001
+#define dm_target_needs_singleton(type)	((type)->features & DM_TARGET_SINGLETON)
+
 struct dm_target {
 	struct dm_table *table;
 	struct target_type *type;