Re: [PATCH v2 mm-new] mm: check if folio has valid mapcount before folio_test_{anon,ksm}() when necessary

[David Hildenbrand <david@redhat.com>]

>   	if (folio_test_anon(folio)) {
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 970600d79dac..00ba2c8b221e 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -706,12 +706,15 @@ PAGEFLAG_FALSE(VmemmapSelfHosted, vmemmap_self_hosted)
>    * address_space which maps the folio from disk; whereas "folio_mapped"
>    * refers to user virtual address space into which the folio is mapped.
>    *
> - * For slab pages, since slab reuses the bits in struct page to store its
> - * internal states, the folio->mapping does not exist as such, nor do
> - * these flags below.  So in order to avoid testing non-existent bits,
> - * please make sure that folio_test_slab(folio) actually evaluates to
> - * false before calling the following functions (e.g., folio_test_anon).
> - * See mm/slab.h.
> + * For certain typed pages like slabs, since they reuse bits in struct page
> + * to store internal states, folio->mapping does not point to a valid
> + * mapping, nor do these flags exist. To avoid testing non-existent bits,
> + * make sure folio_has_mapcount() actually evaluates to true before calling
> + * the following functions (e.g., folio_test_anon).
> + *
> + * The folio_has_mapcount() check can be skipped if the folio is mapped
> + * to userspace, since a folio with !folio_has_mapcount() cannot be mapped
> + * to userspace at all.
>    */
>   #define FOLIO_MAPPING_ANON	0x1
>   #define FOLIO_MAPPING_ANON_KSM	0x2
> @@ -1092,6 +1095,11 @@ static inline bool PageHuge(const struct page *page)
>   	return folio_test_hugetlb(page_folio(page));
>   }
>   
> +static inline bool folio_has_mapcount(const struct folio *folio)
> +{
> +	return !page_has_type(&folio->page) || folio_test_hugetlb(folio);
> +}
> +

I don't like the naming.

The thing is, in the future only folios will have a mapcount.

Asking whether a folio has a mapcount is asking the wrong question.

It's rather, that we want to reject *pages* early, and not cast them to 
the folio in the first place.

-- 
Cheers,

David / dhildenb