PUD_TABLE_ORDER
struct ptdesc *ptdesc = pagetable_alloc(GFP_KERNEL, PUD_TABLE_ORDER);
#define PTRS_PER_PUD ((PAGE_SIZE << PUD_TABLE_ORDER) / sizeof(pud_t))
#define PGDIR_SHIFT (PUD_SHIFT + (PAGE_SHIFT + PUD_TABLE_ORDER - 3))