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 81cbbf375bd7..2ec3482e942a 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;