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Subject: Re: [PATCH 3/3] mm/hmm: Fix bad subpage pointer in try_to_unmap_one
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References: <20190717001446.12351-1-rcampbell@nvidia.com>
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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:
>=20
> BUG: unable to handle page fault for address: ffffea1fffffffc8
>=20
> Currently, only single pages can be migrated to device private memory so
> no subpage computation is needed and it can be set to "page".
>=20
> Fixes: a5430dda8a3a1c ("mm/migrate: support un-addressable ZONE_DEVICE pa=
ge in migration")
> Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
> Cc: "J=C3=A9r=C3=B4me Glisse" <jglisse@redhat.com>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Jason Gunthorpe <jgg@mellanox.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
>  mm/rmap.c | 1 +
>  1 file changed, 1 insertion(+)
>=20
> 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, str=
uct vm_area_struct *vma,
>  			 * No need to invalidate here it will synchronize on
>  			 * against the special swap migration pte.
>  			 */
> +			subpage =3D page;
>  			goto discard;
>  		}

The problem is clear, but the solution still leaves the code ever so slight=
ly
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,
--=20
John Hubbard
NVIDIA