* pte_entry(), and/or hugetlb_entry(). If you don't set up for some of these
* callbacks, the associated entries/pages are just ignored.
* The return values of these callbacks are commonly defined like below:
+ *
* - 0 : succeeded to handle the current entry, and if you don't reach the
* end address yet, continue to walk.
* - >0 : succeeded to handle the current entry, and return to the caller