Re: [PATCH v1 02/12] mm/rmap: reject hugetlb folios in folio_make_device_exclusive()

[Alistair Popple <apopple@nvidia.com>]

On Wed, Jan 29, 2025 at 12:54:00PM +0100, David Hildenbrand wrote:
> Even though FOLL_SPLIT_PMD on hugetlb now always fails with -EOPNOTSUPP,
> let's add a safety net in case FOLL_SPLIT_PMD usage would ever be reworked.
> 
> In particular, before commit 9cb28da54643 ("mm/gup: handle hugetlb in the
> generic follow_page_mask code"), GUP(FOLL_SPLIT_PMD) would just have
> returned a page. In particular, hugetlb folios that are not PMD-sized
> would never have been prone to FOLL_SPLIT_PMD.
> 
> hugetlb folios can be anonymous, and page_make_device_exclusive_one() is
> not really prepared for handling them at all. So let's spell that out.

Agreed, thanks for fixing.

Reviewed-by: Alistair Popple <apopple@nvidia.com>

> Fixes: b756a3b5e7ea ("mm: device exclusive memory access")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  mm/rmap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index c6c4d4ea29a7..17fbfa61f7ef 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -2499,7 +2499,7 @@ static bool folio_make_device_exclusive(struct folio *folio,
>  	 * Restrict to anonymous folios for now to avoid potential writeback
>  	 * issues.
>  	 */
> -	if (!folio_test_anon(folio))
> +	if (!folio_test_anon(folio) || folio_test_hugetlb(folio))
>  		return false;
>  
>  	rmap_walk(folio, &rwc);
> -- 
> 2.48.1
>