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

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Message-ID: <baa1b6cf-2fde-4149-8cdf-4b54e2d7c60d@lucifer.local>
Date: Thu, 28 Aug 2025 16:10:23 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, Zi Yan <ziy@...dia.com>,
        "Mike Rapoport (Microsoft)" <rppt@...nel.org>,
        Alexander Potapenko <glider@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Brendan Jackman <jackmanb@...gle.com>,
        Christoph Lameter <cl@...two.org>, Dennis Zhou <dennis@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>, dri-devel@...ts.freedesktop.org,
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        John Hubbard <jhubbard@...dia.com>, kasan-dev@...glegroups.com,
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Subject: 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 <ziy@...dia.com>
> Acked-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
> Signed-off-by: David Hildenbrand <david@...hat.com>

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 <lorenzo.stoakes@...cle.com>

> ---
>  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
>