bset_prev_bytes
if (bset_prev_bytes(b) < PAGE_SIZE)
get_order(bset_prev_bytes(b)));
t->prev = bset_prev_bytes(b) < PAGE_SIZE
? kmalloc(bset_prev_bytes(b), gfp)
: (void *) __get_free_pages(gfp, get_order(bset_prev_bytes(b)));