I just noticed that bio_clone copies the BIO_SEG_VALID bit from the
original bio when it was set. When we modify bi_idx or bi_vcnt
afterwards the segment counts are invalid and the bit must be dropped
(though it is fairly unlikely that it has already been set).
[Christophe Saout]
--- diff/drivers/md/dm.c	2004-02-09 10:43:40.000000000 +0000
+++ source/drivers/md/dm.c	2004-02-09 12:06:35.000000000 +0000
@@ -409,6 +409,7 @@
 	clone->bi_idx = idx;
 	clone->bi_vcnt = idx + bv_count;
 	clone->bi_size = to_bytes(len);
+	clone->bi_flags &= ~(1 << BIO_SEG_VALID);
 
 	return clone;
 }