dm: pass NULL bdev to bio_alloc_clone

Most DM targets will remap the clone bio passed to their ->map
function using bio_set_bdev(). So this change to pass NULL bdev to
bio_alloc_clone avoids clone-time work that sets up resources for a
bdev association that will not be used in practice (e.g. clone issued
to underlying device will not use DM device's blk-cgroups resources).

But clone->bi_bdev is still initialized following bio_alloc_clone to
preserve DM target expectations that clone->bi_bdev will be set.
Follow-up work is needed to audit DM targets to remove accesses to a
clone->bi_bdev that the target didn't initialize with bio_set_dev().

Depends-on: 7ecc56c6 ("block: allow passing a NULL bdev to bio_alloc_clone/bio_init_clone")
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm.c

@@ -581,7 +581,9 @@ static struct dm_io *alloc_io(struct mapped_device *md, struct bio *bio)
 	struct dm_target_io *tio;
 	struct bio *clone;
 
-	clone = bio_alloc_clone(bio->bi_bdev, bio, GFP_NOIO, &md->io_bs);
+	clone = bio_alloc_clone(NULL, bio, GFP_NOIO, &md->io_bs);
+	/* Set default bdev, but target must bio_set_dev() before issuing IO */
+	clone->bi_bdev = md->disk->part0;
 
 	tio = clone_to_tio(clone);
 	tio->flags = 0;
@@ -624,10 +626,13 @@ static struct bio *alloc_tio(struct clone_info *ci, struct dm_target *ti,
 		/* alloc_io() already initialized embedded clone */
 		clone = &tio->clone;
 	} else {
-		clone = bio_alloc_clone(ci->bio->bi_bdev, ci->bio,
-					gfp_mask, &ci->io->md->bs);
+		struct mapped_device *md = ci->io->md;
+
+		clone = bio_alloc_clone(NULL, ci->bio, gfp_mask, &md->bs);
 		if (!clone)
 			return NULL;
+		/* Set default bdev, but target must bio_set_dev() before issuing IO */
+		clone->bi_bdev = md->disk->part0;
 
 		/* REQ_DM_POLL_LIST shouldn't be inherited */
 		clone->bi_opf &= ~REQ_DM_POLL_LIST;
@@ -1012,13 +1017,15 @@ static bool swap_bios_limit(struct dm_target *ti, struct bio *bio)
 static void clone_endio(struct bio *bio)
 {
 	blk_status_t error = bio->bi_status;
-	struct request_queue *q = bio->bi_bdev->bd_disk->queue;
 	struct dm_target_io *tio = clone_to_tio(bio);
 	struct dm_target *ti = tio->ti;
 	dm_endio_fn endio = ti->type->end_io;
 	struct dm_io *io = tio->io;
 	struct mapped_device *md = io->md;
 
+	if (likely(bio->bi_bdev != md->disk->part0)) {
+		struct request_queue *q = bdev_get_queue(bio->bi_bdev);
+
 		if (unlikely(error == BLK_STS_TARGET)) {
 			if (bio_op(bio) == REQ_OP_DISCARD &&
 			    !bdev_max_discard_sectors(bio->bi_bdev))
@@ -1031,6 +1038,7 @@ static void clone_endio(struct bio *bio)
 		if (static_branch_unlikely(&zoned_enabled) &&
 		    unlikely(blk_queue_is_zoned(q)))
 			dm_zone_endio(io, bio);
+	}
 
 	if (endio) {
 		int r = endio(ti, bio, &error);