linux-kernel - Re: [PATCH v2 2/3] mm/hmm: fix ZONE

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Date:   Mon, 22 Jul 2019 11:38:22 +0200
From:   Christoph Hellwig <hch@....de>
To:     Ralph Campbell <rcampbell@...dia.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Jan Kara <jack@...e.cz>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Jérôme Glisse <jglisse@...hat.com>
Subject: 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@....de>