Man page 9 : bio_copy_data

bio_copy_data - copy contents of data buffers from one chain of bios to another

void bio_copy_data(struct bio *  dst , struct bio *  src );

dst
    destination bio list

src
    source bio list

If src and dst are single bios, bi_next must be NULL - otherwise, treats src and dst as linked lists of bios.

Stops when it reaches the end of either src or dst - that is, copies min(src->bi_size, dst->bi_size) bytes (or the equivalent for lists of bios).