PxD_FLAG_VALID
set_pud(pud, __pud((PxD_FLAG_PRESENT | PxD_FLAG_VALID) +
set_pmd(pmd, __pmd((PxD_FLAG_PRESENT | PxD_FLAG_VALID)
#define pmd_bad(x) (!(pmd_flag(x) & PxD_FLAG_VALID))
#define pud_bad(x) (!(pud_flag(x) & PxD_FLAG_VALID))