__pmd_alloc
int __pmd_alloc(struct mm_struct *mm, pud_t *pud, unsigned long address);
return (unlikely(pud_none(*pud)) && __pmd_alloc(mm, pud, address))?
err = __pmd_alloc(walk->mm, pud, addr);
if (__pmd_alloc(mm, pud, address))