Re: [PATCH v1 11/36] mm: limit folio/compound page sizes in problematic kernel configs

On Thu, Aug 28, 2025 at 12:01:15AM +0200, David Hildenbrand wrote:
> Let's limit the maximum folio size in problematic kernel config where
> the memmap is allocated per memory section (SPARSEMEM without
> SPARSEMEM_VMEMMAP) to a single memory section.
>
> Currently, only a single architectures supports ARCH_HAS_GIGANTIC_PAGE
> but not SPARSEMEM_VMEMMAP: sh.
>
> Fortunately, the biggest hugetlb size sh supports is 64 MiB
> (HUGETLB_PAGE_SIZE_64MB) and the section size is at least 64 MiB
> (SECTION_SIZE_BITS == 26), so their use case is not degraded.
>
> As folios and memory sections are naturally aligned to their order-2 size
> in memory, consequently a single folio can no longer span multiple memory
> sections on these problematic kernel configs.
>
> nth_page() is no longer required when operating within a single compound
> page / folio.
>
> Reviewed-by: Zi Yan <[email protected]>
> Acked-by: Mike Rapoport (Microsoft) <[email protected]>
> Signed-off-by: David Hildenbrand <[email protected]>

Realy great comments, like this!

I wonder if we could have this be part of the first patch where you fiddle
with MAX_FOLIO_ORDER etc. but not a big deal.

Anyway LGTM, so:

Reviewed-by: Lorenzo Stoakes <[email protected]>

> ---
>  include/linux/mm.h | 22 ++++++++++++++++++----
>  1 file changed, 18 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 77737cbf2216a..2dee79fa2efcf 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2053,11 +2053,25 @@ static inline long folio_nr_pages(const struct folio *folio)
>  	return folio_large_nr_pages(folio);
>  }
>
> -/* Only hugetlbfs can allocate folios larger than MAX_ORDER */
> -#ifdef CONFIG_ARCH_HAS_GIGANTIC_PAGE
> -#define MAX_FOLIO_ORDER		PUD_ORDER
> -#else
> +#if !defined(CONFIG_ARCH_HAS_GIGANTIC_PAGE)
> +/*
> + * We don't expect any folios that exceed buddy sizes (and consequently
> + * memory sections).
> + */
>  #define MAX_FOLIO_ORDER		MAX_PAGE_ORDER
> +#elif defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
> +/*
> + * Only pages within a single memory section are guaranteed to be
> + * contiguous. By limiting folios to a single memory section, all folio
> + * pages are guaranteed to be contiguous.
> + */
> +#define MAX_FOLIO_ORDER		PFN_SECTION_SHIFT

Hmmm, was this implicit before somehow? I mean surely by the fact as you say
that physical contiguity would not otherwise be guaranteed :))

> +#else
> +/*
> + * There is no real limit on the folio size. We limit them to the maximum we
> + * currently expect (e.g., hugetlb, dax).
> + */

This is nice.

> +#define MAX_FOLIO_ORDER		PUD_ORDER
>  #endif
>
>  #define MAX_FOLIO_NR_PAGES	(1UL << MAX_FOLIO_ORDER)
> --
> 2.50.1
>