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, 16 insertions, 0 deletions

diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index 81cbbf375bd..2ec3482e942 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -55,6 +55,7 @@ struct dm_table {
 	struct dm_target *targets;
 
 	unsigned integrity_supported:1;
+	unsigned singleton:1;
 
 	/*
 	 * Indicates the rw permissions for the new logical
@@ -740,6 +741,12 @@ int dm_table_add_target(struct dm_table *t, const char *type,
 	char **argv;
 	struct dm_target *tgt;
 
+	if (t->singleton) {
+		DMERR("%s: target type %s must appear alone in table",
+		      dm_device_name(t->md), t->targets->type->name);
+		return -EINVAL;
+	}
+
 	if ((r = check_space(t)))
 		return r;
 
@@ -758,6 +765,15 @@ int dm_table_add_target(struct dm_table *t, const char *type,
 		return -EINVAL;
 	}
 
+	if (dm_target_needs_singleton(tgt->type)) {
+		if (t->num_targets) {
+			DMERR("%s: target type %s must appear alone in table",
+			      dm_device_name(t->md), type);
+			return -EINVAL;
+		}
+		t->singleton = 1;
+	}
+
 	tgt->table = t;
 	tgt->begin = start;
 	tgt->len = len;