__p4d_alloc
int __p4d_alloc(struct mm_struct *mm, pgd_t *pgd, unsigned long address);
return (unlikely(pgd_none(*pgd)) && __p4d_alloc(mm, pgd, address)) ?
err = __p4d_alloc(walk->mm, pgd, addr);
if (__p4d_alloc(mm, pgd, address))