Re: [PATCH 2/3] mm/hmm: fix ZONE_DEVICE anon page mapping reuse

[John Hubbard]

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@xxxxxxxxxxxxxxx
> Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
> Cc: Logan Gunthorpe <logang@xxxxxxxxxxxx>
> Cc: Ira Weiny <ira.weiny@xxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Cc: "JÃrÃme Glisse" <jglisse@xxxxxxxxxx>
> ---
>  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