dm: allow dm_accept_partial_bio() for dm_io without duplicate bios

The intent behind commit e6fc9f6 ("dm: flag clones created by
__send_duplicate_bios") was to formally disallow the use of
dm_accept_partial_bio() where it simply isn't possible -- due to
constraint that multiple bios cannot meaningfully update a shared
tio->len_ptr.

But that commit went too far and disallowed the case where "abormal"
IO (e.g. WRITE_ZEROES) is only using a single bio.  Fix this by
not marking a dm_io with a single dm_target_io (and bio), that happens
to be created by __send_duplicate_bios, as DM_TIO_IS_DUPLICATE_BIO.
Also remove 'unsigned *len' parameter from alloc_multiple_bios().

This commit fixes a dm_accept_partial_bio() BUG_ON() with dm-zoned
when a WRITE_ZEROES bio is issued.

Fixes: 655f3aa ("dm: switch dm_target_io booleans over to proper flags")
Reported-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

Author: Mike Snitzer

drivers/md/dm.c

@@ -1323,8 +1323,7 @@ static void __map_bio(struct bio *clone)
 }
 
 static void alloc_multiple_bios(struct bio_list *blist, struct clone_info *ci,
-				struct dm_target *ti, unsigned num_bios,
-				unsigned *len)
+				struct dm_target *ti, unsigned num_bios)
 {
 	struct bio *bio;
 	int try;
@@ -1335,7 +1334,7 @@ static void alloc_multiple_bios(struct bio_list *blist, struct clone_info *ci,
 		if (try)
 			mutex_lock(&ci->io->md->table_devices_lock);
 		for (bio_nr = 0; bio_nr < num_bios; bio_nr++) {
-			bio = alloc_tio(ci, ti, bio_nr, len,
+			bio = alloc_tio(ci, ti, bio_nr, NULL,
 					try ? GFP_NOIO : GFP_NOWAIT);
 			if (!bio)
 				break;
@@ -1363,11 +1362,11 @@ static void __send_duplicate_bios(struct clone_info *ci, struct dm_target *ti,
 		break;
 	case 1:
 		clone = alloc_tio(ci, ti, 0, len, GFP_NOIO);
-		dm_tio_set_flag(clone_to_tio(clone), DM_TIO_IS_DUPLICATE_BIO);
 		__map_bio(clone);
 		break;
 	default:
-		alloc_multiple_bios(&blist, ci, ti, num_bios, len);
+		/* dm_accept_partial_bio() is not supported with shared tio->len_ptr */
+		alloc_multiple_bios(&blist, ci, ti, num_bios);
 		while ((clone = bio_list_pop(&blist))) {
 			dm_tio_set_flag(clone_to_tio(clone), DM_TIO_IS_DUPLICATE_BIO);
 			__map_bio(clone);
@@ -1407,14 +1406,10 @@ static void __send_changing_extent_only(struct clone_info *ci, struct dm_target
 	len = min_t(sector_t, ci->sector_count,
 		    max_io_len_target_boundary(ti, dm_target_offset(ti, ci->sector)));
 
-	/*
-	 * dm_accept_partial_bio cannot be used with duplicate bios,
-	 * so update clone_info cursor before __send_duplicate_bios().
-	 */
+	__send_duplicate_bios(ci, ti, num_bios, &len);
+
 	ci->sector += len;
 	ci->sector_count -= len;
-
-	__send_duplicate_bios(ci, ti, num_bios, &len);
 }
 
 static bool is_abnormal_io(struct bio *bio)