pmd_modify
extern pmd_t pmd_modify(pmd_t pmd, pgprot_t newprot);
pmd = pmd_modify(old_pmd, vma->vm_page_prot);
pmd = pmd_modify(pmdp_get(vmf->pmd), vma->vm_page_prot);
entry = pmd_modify(oldpmd, newprot);