bh->b_rdev wasn't being set properly.  Bug from earlier patch.
--- diff/drivers/md/dm-raid1.c	2003-10-16 10:45:21.000000000 +0100
+++ source/drivers/md/dm-raid1.c	2003-10-16 10:45:31.000000000 +0100
@@ -720,6 +720,7 @@
 static void map_buffer(struct mirror_set *ms,
 		       struct mirror *m, struct buffer_head *bh)
 {
+	bh->b_rdev = m->dev->dev;
 	bh->b_rsector = m->offset + (bh->b_rsector - ms->ti->begin);
 }