dm table: add always writeable feature

Add a target feature flag DM_TARGET_ALWAYS_WRITEABLE to indicate that a target
does not support read-only mode.

The initial implementation of the thin provisioning target uses this.

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

2 files changed, 13 insertions, 0 deletions

diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index 2ec3482e942a..9917141729ef 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -774,6 +774,12 @@ int dm_table_add_target(struct dm_table *t, const char *type,
                 t->singleton = 1;
         }
 
+        if (dm_target_always_writeable(tgt->type) && !(t->mode & FMODE_WRITE)) {
+                DMERR("%s: target type %s may not be included in read-only tables",
+                      dm_device_name(t->md), type);
+                return -EINVAL;
+        }
+
         tgt->table = t;
         tgt->begin = start;
         tgt->len = len;
diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
index 294e78a7fccd..cc58e2d7c032 100644
--- a/include/linux/device-mapper.h
+++ b/include/linux/device-mapper.h
@@ -166,6 +166,13 @@ struct target_type {
 #define DM_TARGET_SINGLETON             0x00000001
 #define dm_target_needs_singleton(type) ((type)->features & DM_TARGET_SINGLETON)
 
+/*
+ * Indicates that a target does not support read-only devices.
+ */
+#define DM_TARGET_ALWAYS_WRITEABLE      0x00000002
+#define dm_target_always_writeable(type) \
+                ((type)->features & DM_TARGET_ALWAYS_WRITEABLE)
+
 struct dm_target {
         struct dm_table *table;
         struct target_type *type;