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SubjectRe: [PATCH 2/3] mm/hmm: fix ZONE_DEVICE anon page mapping reuse
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On 7/16/19 5:14 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.
>
> Fixes: b7a523109fb5c9d2d6dd ("mm: don't clear ->mapping in hmm_devmem_free")
> Cc: stable@vger.kernel.org
> Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Jason Gunthorpe <jgg@mellanox.com>
> Cc: Logan Gunthorpe <logang@deltatee.com>
> Cc: Ira Weiny <ira.weiny@intel.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: Jan Kara <jack@suse.cz>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> Cc: "Jérôme Glisse" <jglisse@redhat.com>
> ---
> kernel/memremap.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/kernel/memremap.c b/kernel/memremap.c
> index bea6f887adad..238ae5d0ae8a 100644
> --- a/kernel/memremap.c
> +++ b/kernel/memremap.c
> @@ -408,6 +408,10 @@ void __put_devmap_managed_page(struct page *page)
>
> mem_cgroup_uncharge(page);
>
> + /* Clear anonymous page mapping to prevent stale pointers */

This is sufficiently complex, that some concise form of the documentation
that you've put in the commit description, needs to also exist right here, as
a comment.

How's this read:

diff --git a/kernel/memremap.c b/kernel/memremap.c
index 238ae5d0ae8a..e52e9da5d0a7 100644
--- a/kernel/memremap.c
+++ b/kernel/memremap.c
@@ -408,7 +408,27 @@ void __put_devmap_managed_page(struct page *page)

mem_cgroup_uncharge(page);

- /* Clear anonymous page mapping to prevent stale pointers */
+ /*
+ * 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 an 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,
--
John Hubbard
NVIDIA

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