Re: [PATCH v2 2/3] mm/hmm: fix ZONE_DEVICE anon page mapping reuse

• /*
  

•  * 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@lst.de