dm: eliminate unused "bioset" process for each bio-based DM device

Commit 54efd50bfd873e2dbf784e0b21a8027ba4299a3e ("block: make
generic_make_request handle arbitrarily sized bios") makes it possible
for block devices to process large bios.  In doing so that commit
allocates a new queue->bio_split bioset for each block device, this
bioset is used for allocating bios when the driver needs to split large
bios.

Each bioset allocates a workqueue process, thus the above commit
increases the number of processes allocated per block device.

DM doesn't need the queue->bio_split bioset, thus we can deallocate it.
This reduces the number of allocated processes per bio-based DM device
from 3 to 2.  Also remove the call to blk_queue_split(), it is not
needed because DM does its own splitting.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm.c

@@ -1763,8 +1763,6 @@ static void dm_make_request(struct request_queue *q, struct bio *bio)
 
 	map = dm_get_live_table(md, &srcu_idx);
 
-	blk_queue_split(q, &bio, q->bio_split);
-
 	generic_start_io_acct(rw, bio_sectors(bio), &dm_disk(md)->part0);
 
 	/* if we're suspended, we have to queue this io for later */
@@ -2792,6 +2790,12 @@ int dm_setup_md_queue(struct mapped_device *md)
 	case DM_TYPE_BIO_BASED:
 		dm_init_old_md_queue(md);
 		blk_queue_make_request(md->queue, dm_make_request);
+		/*
+		 * DM handles splitting bios as needed.  Free the bio_split bioset
+		 * since it won't be used (saves 1 process per bio-based DM device).
+		 */
+		bioset_free(md->queue->bio_split);
+		md->queue->bio_split = NULL;
 		break;
 	}