+ *
+ * And we can skip it on file pages, so long as the filesystem
+ * participates in dirty tracking; but need to catch shm and tmpfs
+ * and ramfs pages which have been modified since creation by read
+ * fault.
+ *
+ * Note that mapping must be decided above, before decrementing
+ * mapcount (which luckily provides a barrier): once page is unmapped,
+ * it could be truncated and page->mapping reset to NULL at any moment.
+ * Note also that we are relying on page_mapping(page) to set mapping
+ * to &swapper_space when PageSwapCache(page).