Re: [PATCH 3/3] mm/hmm: Fix bad subpage pointer in try_to_unmap_one

[John Hubbard]

On 7/16/19 5:14 PM, Ralph Campbell wrote:
> When migrating an anonymous private page to a ZONE_DEVICE private page,
> the source page->mapping and page->index fields are copied to the
> destination ZONE_DEVICE struct page and the page_mapcount() is increased.
> This is so rmap_walk() can be used to unmap and migrate the page back to
> system memory. However, try_to_unmap_one() computes the subpage pointer
> from a swap pte which computes an invalid page pointer and a kernel panic
> results such as:
> 
> BUG: unable to handle page fault for address: ffffea1fffffffc8
> 
> Currently, only single pages can be migrated to device private memory so
> no subpage computation is needed and it can be set to "page".
> 
> Fixes: a5430dda8a3a1c ("mm/migrate: support un-addressable ZONE_DEVICE page in migration")
> Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
> Cc: "JÃrÃme Glisse" <jglisse@xxxxxxxxxx>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> ---
>  mm/rmap.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index e5dfe2ae6b0d..ec1af8b60423 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1476,6 +1476,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
>  			 * No need to invalidate here it will synchronize on
>  			 * against the special swap migration pte.
>  			 */
> +			subpage = page;
>  			goto discard;
>  		}

The problem is clear, but the solution still leaves the code ever so slightly
more confusing, and it was already pretty difficult to begin with.

I still hold out hope for some comment documentation at least, and maybe
even just removing the subpage variable (as Jerome mentioned, offline) as
well.

Jerome?


thanks,
-- 
John Hubbard
NVIDIA