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 <jhubb...@nvidia.com>
thanks,
--
John Hubbard
NVIDIA
>
> Fixes: b7a523109fb5c9d2d6dd ("mm: don't clear ->mapping in hmm_devmem_free")
> Cc: sta...@vger.kernel.org
> Signed-off-by: Ralph Campbell <rcampb...@nvidia.com>
> Cc: Christoph Hellwig <h...@lst.de>
> Cc: Dan Williams <dan.j.willi...@intel.com>
> Cc: Andrew Morton <a...@linux-foundation.org>
> Cc: Jason Gunthorpe <j...@mellanox.com>
> Cc: Logan Gunthorpe <log...@deltatee.com>
> Cc: Ira Weiny <ira.we...@intel.com>
> Cc: Matthew Wilcox <wi...@infradead.org>
> Cc: Mel Gorman <mgor...@techsingularity.net>
> Cc: Jan Kara <j...@suse.cz>
> Cc: "Kirill A. Shutemov" <kirill.shute...@linux.intel.com>
> Cc: Michal Hocko <mho...@suse.com>
> Cc: Andrea Arcangeli <aarca...@redhat.com>
> Cc: Mike Kravetz <mike.krav...@oracle.com>
> Cc: "Jérôme Glisse" <jgli...@redhat.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);
>
