When a bio is submitted to the block-layer, several of the fields in that bio
will have changed by the time it completes. In particular, the bi_sector
field will be different, the bi_size field will be zero, and the bi_bdev and
bi_idx fields may have changed as well. This creates a problem for some
of the Device-Mapper modules (like dm-multipath) that may want to redrive a
bio.

Save the necessary bio fields in the target_io structure just before calling
generic_make_request(), and restore those bio fields just before calling the
target's end_io() routine. This ensures that the target module sees the bio
exactly as it was following the map() routine.  [Kevin Corry]
--- diff/drivers/md/dm.c	2004-01-16 13:57:42.000000000 +0000
+++ source/drivers/md/dm.c	2004-01-16 17:20:37.000000000 +0000
@@ -42,6 +42,11 @@
 	struct dm_io *io;
 	struct dm_target *ti;
 	union map_info info;
+
+	sector_t bi_sector;
+	struct block_device *bi_bdev;
+	unsigned int bi_size;
+	unsigned short bi_idx;
 };
 
 /*
@@ -276,13 +281,19 @@
 		return 1;
 
 	if (endio) {
+		/* Restore bio fields. */
+		bio->bi_sector = tio->bi_sector;
+		bio->bi_bdev = tio->bi_bdev;
+		bio->bi_size = tio->bi_size;
+		bio->bi_idx = tio->bi_idx;
+
 		r = endio(tio->ti, bio, error, &tio->info);
 		if (r < 0)
 			error = r;
 
 		else if (r > 0)
 			/* the target wants another shot at the io */
-			return 1; /* FIXME: do we need to reset bio at all ? */
+			return 1;
 	}
 
 	free_tio(io->md, tio);
@@ -331,9 +342,16 @@
 	 */
 	atomic_inc(&tio->io->io_count);
 	r = ti->type->map(ti, clone, &tio->info);
-	if (r > 0)
+	if (r > 0) {
+		/* Save the bio info so we can restore it during endio. */
+		tio->bi_sector = clone->bi_sector;
+		tio->bi_bdev = clone->bi_bdev;
+		tio->bi_size = clone->bi_size;
+		tio->bi_idx = clone->bi_idx;
+
 		/* the bio has been remapped so dispatch it */
 		generic_make_request(clone);
+	}
 
 	else if (r < 0) {
 		/* error the io and bail out */