dm: allow targets to request flushes regardless of underlying device support

Allow targets to override the 'supports flush' calculation.

Set 'flush_supported' if a target needs to receive flushes regardless of
whether or not its underlying devices have support.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm-table.c

@@ -1319,6 +1319,9 @@ static bool dm_table_supports_flush(struct dm_table *t, unsigned flush)
 		if (!ti->num_flush_requests)
 			continue;
 
+		if (ti->flush_supported)
+			return 1;
+
 		if (ti->type->iterate_devices &&
 		    ti->type->iterate_devices(ti, device_flush_capable, &flush))
 			return 1;

include/linux/device-mapper.h

@@ -211,6 +211,12 @@ struct dm_target {
 	/* Used to provide an error string from the ctr */
 	char *error;
 
+	/*
+	 * Set if this target needs to receive flushes regardless of
+	 * whether or not its underlying devices have support.
+	 */
+	bool flush_supported:1;
+
 	/*
 	 * Set if this target needs to receive discards regardless of
 	 * whether or not its underlying devices have support.