pte_t *
pgdir_walk(pde_t *pgdir, const void *va, int create)
{
	pde_t page_dir_entry = pgdir[PDX(va)];
	if (!page_dir_entry && create) {
		struct PageInfo *page = page_alloc(ALLOC_ZERO);
		if (page != NULL) {
			page->pp_ref += 1;
			page_dir_entry = PTE_ADDR(page2pa(page)) | PTE_SYSCALL;
			pgdir[PDX(va)] = page_dir_entry;
		}
	}
	if (!page_dir_entry) {
		return NULL;
	}

	pte_t *page_table = KADDR(PTE_ADDR(page_dir_entry));
	return page_table + PTX(va);
}