/* submap and unsubmap flags indicate:
* 00: subregion has never been mmaped
* 10: subregion has been mmaped, reference to the mm was taken
* 11: subretion has ben released, refernece to the mm still held
* 01: subretion has been released, reference to the mm has been released
*/
/* submap and unsubmap flags indicate:
* 00: subregion has never been mmaped
* 10: subregion has been mmaped, reference to the mm was taken
* 11: subretion has ben released, refernece to the mm still held
* 01: subretion has been released, reference to the mm has been released
*/