flush_bios
struct bio_list flush_bios = BIO_EMPTY_LIST;
bio_list_add(&flush_bios, bio);
bio_list_merge(&clone->deferred_flush_bios, &flush_bios);
flush_bios(bio_list_get(&flush_bio_list));
flush_bios(b);
flush_bios(snapshot_bios);
static void flush_bios(struct bio *bio);