Re: [PATCH v1 25/29] mm: simplify folio_expected_ref_count()

[Lorenzo Stoakes]

On Mon, Jun 30, 2025 at 03:00:06PM +0200, David Hildenbrand wrote:
> Now that PAGE_MAPPING_MOVABLE is gone, we can simplify and rely on the
> folio_test_anon() test only.
>
> ... but staring at the users, this function should never even have been
> called on movable_ops pages. E.g.,
> * __buffer_migrate_folio() does not make sense for them
> * folio_migrate_mapping() does not make sense for them
> * migrate_huge_page_move_mapping() does not make sense for them
> * __migrate_folio() does not make sense for them
> * ... and khugepaged should never stumble over them
>
> Let's simply refuse typed pages (which includes slab) except hugetlb,
> and WARN.

I guess also:

* PGTY_buddy - raw buddy allocator pagess should't be here...
* PGTY_table - nor page table...
* PGTY_guard - nor whatever kind of guard this is I assume? (Not my precious guard regions :P)
* PGTY_unaccepted - nor unaccepted memory perhaps?
* PGTY_large_malloc - slab, shouldn't be here

I'd maybe delineate these cases also.

>
> Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

On assumption no typed page should be tolerable here:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>

> ---
>  include/linux/mm.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 6a5447bd43fd8..f6ef4c4eb536b 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2176,13 +2176,13 @@ static inline int folio_expected_ref_count(const struct folio *folio)
>  	const int order = folio_order(folio);
>  	int ref_count = 0;
>
> -	if (WARN_ON_ONCE(folio_test_slab(folio)))
> +	if (WARN_ON_ONCE(page_has_type(&folio->page) && !folio_test_hugetlb(folio)))
>  		return 0;
>
>  	if (folio_test_anon(folio)) {
>  		/* One reference per page from the swapcache. */
>  		ref_count += folio_test_swapcache(folio) << order;
> -	} else if (!((unsigned long)folio->mapping & PAGE_MAPPING_FLAGS)) {
> +	} else {
>  		/* One reference per page from the pagecache. */
>  		ref_count += !!folio->mapping << order;
>  		/* One reference from PG_private. */
> --
> 2.49.0
>