dm table: allow targets to support discards internally

Permit a target to support discards regardless of whether or not all its
underlying devices do.
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm-table.c

@@ -1346,7 +1346,8 @@ bool dm_table_supports_discards(struct dm_table *t)
 		return 0;
 
 	/*
-	 * Ensure that at least one underlying device supports discards.
+	 * Unless any target used by the table set discards_supported,
+	 * require at least one underlying device to support discards.
 	 * t->devices includes internal dm devices such as mirror logs
 	 * so we need to use iterate_devices here, which targets
 	 * supporting discard must provide.
@@ -1354,6 +1355,9 @@ bool dm_table_supports_discards(struct dm_table *t)
 	while (i < dm_table_get_num_targets(t)) {
 		ti = dm_table_get_target(t, i++);
 
+		if (ti->discards_supported)
+			return 1;
+
 		if (ti->type->iterate_devices &&
 		    ti->type->iterate_devices(ti, device_discard_capable, NULL))
 			return 1;

include/linux/device-mapper.h

@@ -191,6 +191,12 @@ struct dm_target {
 
 	/* Used to provide an error string from the ctr */
 	char *error;
+
+	/*
+	 * Set if this target needs to receive discards regardless of
+	 * whether or not its underlying devices have support.
+	 */
+	unsigned discards_supported:1;
 };
 
 /* Each target can link one of these into the table */