dm

 //The request function that just remaps the bio built up by dm_merge_bvec

static int dm_request(struct request_queue *q, struct bio *bio)

       call  __split_bio

// Split the bio into several clones.

static int __split_bio(struct mapped_device *md, struct bio *bio)

       call  __clone_and_map with parameter ci

       ( ci.sector_count= bio_sectors(bio)  )

static int __clone_and_map(struct clone_info *ci)

call  split_bvec