The mapcount field was created to track the number of mappings (page-table entries) that refer to the given page. Among other things, a mapcount of zero means that the page has no references and can be reclaimed. Maintaining mapcount has become increasingly challenging and expensive as the memory-management system has grown in complexity, so Hildenbrand has been looking for ways to get rid of it. This session was, he said, maybe one of the last times he will have to bring up this topic.