pmdp_xchg_lazy
pmd_t pmdp_xchg_lazy(struct mm_struct *, unsigned long, pmd_t *, pmd_t);
pmd = pmdp_xchg_lazy(vma->vm_mm, addr, pmdp, __pmd(_SEGMENT_ENTRY_EMPTY));
pmd = pmdp_xchg_lazy(mm, addr, pmdp, pmd_wrprotect(pmd));
EXPORT_SYMBOL(pmdp_xchg_lazy);