subsection_map_index
int idx = subsection_map_index(pfn);
int idx = subsection_map_index(*pfn);
int idx = subsection_map_index(pfn);
int end = subsection_map_index(pfn + nr_pages - 1);
pfns, subsection_map_index(pfn),
subsection_map_index(pfn + pfns - 1));