Man page 9 : blk_rq_prep_clone

blk_rq_prep_clone - Helper function to setup clone request

int blk_rq_prep_clone(struct request *  rq , struct request *  rq_src , struct bio_set *  bs , gfp_t  gfp_mask , int (* bio_ctr ) (struct bio *, struct bio *, void *), void *  data );

rq
    the request to be setup

rq_src
    original request to be cloned

bs
    bio_set that bios for clone are allocated from

gfp_mask
    memory allocation mask for bio

bio_ctr
    setup function to be called for each clone bio. Returns 0 for success, non 0 for failure.

data
    private data to be passed to bio_ctr

Clones bios in rq_src to rq, and copies attributes of rq_src to rq. The actual data parts of rq_src (e.g. ->cmd, ->buffer, ->sense) are not copied, and copying such parts is the callerAqs responsibility. Also, pages which the original bios are pointing to are not copied and the cloned bios just point same pages. So cloned bios must be completed before original bios, which means the caller must complete rq before rq_src.