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,