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

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Date:   Fri, 19 Jul 2019 14:45:08 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Ralph Campbell <rcampbell@...dia.com>, <linux-mm@...ck.org>
CC:     <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

On 7/19/19 12:29 PM, Ralph Campbell wrote:
> When a ZONE_DEVICE private page is freed, the page->mapping field can be
> set. If this page is reused as an anonymous page, the previous value can
> prevent the page from being inserted into the CPU's anon rmap table.
> For example, when migrating a pte_none() page to device memory:
>   migrate_vma(ops, vma, start, end, src, dst, private)
>     migrate_vma_collect()
>       src[] = MIGRATE_PFN_MIGRATE
>     migrate_vma_prepare()
>       /* no page to lock or isolate so OK */
>     migrate_vma_unmap()
>       /* no page to unmap so OK */
>     ops->alloc_and_copy()
>       /* driver allocates ZONE_DEVICE page for dst[] */
>     migrate_vma_pages()
>       migrate_vma_insert_page()
>         page_add_new_anon_rmap()
>           __page_set_anon_rmap()
>             /* This check sees the page's stale mapping field */
>             if (PageAnon(page))
>               return
>             /* page->mapping is not updated */
> 
> The result is that the migration appears to succeed but a subsequent CPU
> fault will be unable to migrate the page back to system memory or worse.
> 
> Clear the page->mapping field when freeing the ZONE_DEVICE page so stale
> pointer data doesn't affect future page use.

Reviewed-by: John Hubbard <jhubbard@...dia.com>

thanks,
-- 
John Hubbard
NVIDIA

> 
> Fixes: b7a523109fb5c9d2d6dd ("mm: don't clear ->mapping in hmm_devmem_free")
> Cc: stable@...r.kernel.org
> Signed-off-by: Ralph Campbell <rcampbell@...dia.com>
> Cc: Christoph Hellwig <hch@....de>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Jason Gunthorpe <jgg@...lanox.com>
> Cc: Logan Gunthorpe <logang@...tatee.com>
> Cc: Ira Weiny <ira.weiny@...el.com>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Mel Gorman <mgorman@...hsingularity.net>
> Cc: Jan Kara <jack@...e.cz>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> Cc: Mike Kravetz <mike.kravetz@...cle.com>
> Cc: "Jérôme Glisse" <jglisse@...hat.com>
> ---
>  kernel/memremap.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/kernel/memremap.c b/kernel/memremap.c
> index bea6f887adad..98d04466dcde 100644
> --- a/kernel/memremap.c
> +++ b/kernel/memremap.c
> @@ -408,6 +408,30 @@ void __put_devmap_managed_page(struct page *page)
>  
>  		mem_cgroup_uncharge(page);
>  
> +		/*
> +		 * 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;
> +
>  		page->pgmap->ops->page_free(page);
>  	} else if (!count)
>  		__put_page(page);
>