PBUF_PAGES
maxpages = PBUF_PAGES;
(vm_offset_t)uaddr, len, prot, bp->b_pages, PBUF_PAGES);
KASSERT(bp->b_npages <= PBUF_PAGES,
bp->b_npages, rbehind, count, rahead, (uintmax_t)PBUF_PAGES));
sizeof(struct buf) + PBUF_PAGES * sizeof(vm_page_t),
bp->b_kvabase = kva_alloc(ptoa(PBUF_PAGES));
bp->b_kvasize = ptoa(PBUF_PAGES);