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

[Christoph Hellwig <hch@lst.de>]

> +		/*
> +		 * 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>