> + /* > + * When a device_private page is freed, the page->mapping field > + * may still contain a (stale) mapping value. For example, the > + * lower bits of page->mapping may still identify the page as > + * an anonymous page. Ultimately, this entire field is just > + * stale and wrong, and it will cause errors if not cleared. > + * One example is: > + * > + * migrate_vma_pages() > + * migrate_vma_insert_page() > + * page_add_new_anon_rmap() > + * __page_set_anon_rmap() > + * ...checks page->mapping, via PageAnon(page) call, > + * and incorrectly concludes that the page is an > + * anonymous page. Therefore, it incorrectly, > + * silently fails to set up the new anon rmap. > + * > + * For other types of ZONE_DEVICE pages, migration is either > + * handled differently or not done at all, so there is no need > + * to clear page->mapping. > + */ > + if (is_device_private_page(page)) > + page->mapping = NULL; > + Thanks, especially for the long comment. Reviewed-by: Christoph Hellwig <hch@xxxxxx>
