dm: remove unused 'num_write_bios' target interface

No DM target provides num_write_bios and none has since dm-cache's
brief use in 2013.

Having the possibility of num_write_bios > 1 complicates bio
allocation.  So remove the interface and assume there is only one bio
needed.

If a target ever needs more, it must provide a suitable bioset and
allocate itself based on its particular needs.
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm.c

@@ -1319,32 +1319,22 @@ static int __send_empty_flush(struct clone_info *ci)
 }
 
 static int __clone_and_map_data_bio(struct clone_info *ci, struct dm_target *ti,
-				     sector_t sector, unsigned *len)
+				    sector_t sector, unsigned *len)
 {
 	struct bio *bio = ci->bio;
 	struct dm_target_io *tio;
-	unsigned target_bio_nr;
-	unsigned num_target_bios = 1;
-	int r = 0;
+	int r;
 
-	/*
-	 * Does the target want to receive duplicate copies of the bio?
-	 */
-	if (bio_data_dir(bio) == WRITE && ti->num_write_bios)
-		num_target_bios = ti->num_write_bios(ti, bio);
-
-	for (target_bio_nr = 0; target_bio_nr < num_target_bios; target_bio_nr++) {
-		tio = alloc_tio(ci, ti, target_bio_nr);
-		tio->len_ptr = len;
-		r = clone_bio(tio, bio, sector, *len);
-		if (r < 0) {
-			free_tio(tio);
-			break;
-		}
-		__map_bio(tio);
+	tio = alloc_tio(ci, ti, 0);
+	tio->len_ptr = len;
+	r = clone_bio(tio, bio, sector, *len);
+	if (r < 0) {
+		free_tio(tio);
+		return r;
 	}
+	__map_bio(tio);
 
-	return r;
+	return 0;
 }
 
 typedef unsigned (*get_num_bios_fn)(struct dm_target *ti);

include/linux/device-mapper.h

@@ -220,14 +220,6 @@ struct target_type {
 #define DM_TARGET_WILDCARD		0x00000008
 #define dm_target_is_wildcard(type)	((type)->features & DM_TARGET_WILDCARD)
 
-/*
- * Some targets need to be sent the same WRITE bio severals times so
- * that they can send copies of it to different devices.  This function
- * examines any supplied bio and returns the number of copies of it the
- * target requires.
- */
-typedef unsigned (*dm_num_write_bios_fn) (struct dm_target *ti, struct bio *bio);
-
 /*
  * A target implements own bio data integrity.
  */
@@ -291,13 +283,6 @@ struct dm_target {
 	 */
 	unsigned per_io_data_size;
 
-	/*
-	 * If defined, this function is called to find out how many
-	 * duplicate bios should be sent to the target when writing
-	 * data.
-	 */
-	dm_num_write_bios_fn num_write_bios;
-
 	/* target specific data */
 	void *private;