dm: allow error target to replace bio-based and request-based targets

It may be useful to switch a request-based table to the "error" target.
Enhance the DM core to allow a hybrid target_type which is capable of
handling either bios (via .map) or requests (via .map_rq).

Add a request-based map function (.map_rq) to the "error" target_type;
making it DM's first hybrid target. Train dm_table_set_type() to prefer
the mapped device's established type (request-based or bio-based). If
the mapped device doesn't have an established type default to making the
table with the hybrid target(s) bio-based.

Tested 'dmsetup wipe_table' to work on both bio-based and request-based
devices.

Signed-off-by: Mike Snitzer <[e-mail]>
Signed-off-by: Joe Jin <[e-mail]>
Signed-off-by: Jun'ichi Nomura <[e-mail]>
Acked-by: Mikulas Patocka <[e-mail]>
Signed-off-by: Alasdair G Kergon <[e-mail]>

drivers/md/dm-table.c

···
 static int dm_table_set_type(struct dm_table *t)
 {
 	unsigned i;
-	unsigned bio_based = 0, request_based = 0;
+	unsigned bio_based = 0, request_based = 0, hybrid = 0;
 	struct dm_target *tgt;
 	struct dm_dev_internal *dd;
 	struct list_head *devices;
+	unsigned live_md_type;
 
 	for (i = 0; i < t->num_targets; i++) {
 		tgt = t->targets + i;
-		if (dm_target_request_based(tgt))
+		if (dm_target_hybrid(tgt))
+			hybrid = 1;
+		else if (dm_target_request_based(tgt))
 			request_based = 1;
 		else
 			bio_based = 1;
···
 			       " can't be mixed up");
 			return -EINVAL;
 		}
+	}
+
+	if (hybrid && !bio_based && !request_based) {
+		/*
+		 * The targets can work either way.
+		 * Determine the type from the live device.
+		 * Default to bio-based if device is new.
+		 */
+		dm_lock_md_type(t->md);
+		live_md_type = dm_get_md_type(t->md);
+		dm_unlock_md_type(t->md);
+		if (live_md_type == DM_TYPE_REQUEST_BASED)
+			request_based = 1;
+		else
+			bio_based = 1;
 	}
 
 	if (bio_based) {

drivers/md/dm-target.c

···
 	return -EIO;
 }
 
+static int io_err_map_rq(struct dm_target *ti, struct request *clone,
+			 union map_info *map_context)
+{
+	return -EIO;
+}
+
 static struct target_type error_target = {
 	.name = "error",
-	.version = {1, 1, 0},
+	.version = {1, 2, 0},
 	.ctr  = io_err_ctr,
 	.dtr  = io_err_dtr,
 	.map  = io_err_map,
+	.map_rq = io_err_map_rq,
 };
 
 int __init dm_target_init(void)

drivers/md/dm.h

···
 #define dm_target_is_valid(t) ((t)->table)
 
 /*
+ * To check whether the target type is bio-based or not (request-based).
+ */
+#define dm_target_bio_based(t) ((t)->type->map != NULL)
+
+/*
  * To check whether the target type is request-based or not (bio-based).
  */
 #define dm_target_request_based(t) ((t)->type->map_rq != NULL)
+
+/*
+ * To check whether the target type is a hybrid (capable of being
+ * either request-based or bio-based).
+ */
+#define dm_target_hybrid(t) (dm_target_bio_based(t) && dm_target_request_based(t))
 
 /*-----------------------------------------------------------------
  * A registry of target types.