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@nvidia.com
thanks,
--
John Hubbard
NVIDIA
>
> 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 | 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);
>
