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@nvidia.com>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: David Hildenbrand <david@redhat.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
+#else
+/*
+ * There is no real limit on the folio size. We limit them to the maximum we
+ * currently expect (e.g., hugetlb, dax).
+ */
+#define MAX_FOLIO_ORDER		PUD_ORDER
 #endif
 
 #define MAX_FOLIO_NR_PAGES	(1UL << MAX_FOLIO_ORDER)
-- 
2.50.1

* David Hildenbrand <david@redhat.com> [250827 18:05]:
> 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@nvidia.com>
> Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> Signed-off-by: David Hildenbrand <david@redhat.com>


Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.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
> +#else
> +/*
> + * There is no real limit on the folio size. We limit them to the maximum we
> + * currently expect (e.g., hugetlb, dax).
> + */
> +#define MAX_FOLIO_ORDER		PUD_ORDER
>  #endif
>  
>  #define MAX_FOLIO_NR_PAGES	(1UL << MAX_FOLIO_ORDER)
> -- 
> 2.50.1
>

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@nvidia.com>
> Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> Signed-off-by: David Hildenbrand <david@redhat.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@oracle.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
>

On 28.08.25 17:10, Lorenzo Stoakes wrote:
> 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@nvidia.com>
>> Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
>> Signed-off-by: David Hildenbrand <david@redhat.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.

I think it belongs into this patch where we actually impose the 
restrictions.

[...]

>> +/*
>> + * 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 :))

Well, my patches until this point made sure that any attempt to use a 
larger folio would fail in a way that we could spot now if there is any 
offender.

That is why before this change, nth_page() was required within a folio.

Hope that clarifies it, thanks!

-- 
Cheers

David / dhildenb

On Fri, Aug 29, 2025 at 01:57:22PM +0200, David Hildenbrand wrote:
> On 28.08.25 17:10, Lorenzo Stoakes wrote:
> > 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@nvidia.com>
> > > Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> > > Signed-off-by: David Hildenbrand <david@redhat.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.
>
> I think it belongs into this patch where we actually impose the
> restrictions.

Sure it's not a big deal.

>
> [...]
>
> > > +/*
> > > + * 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 :))
>
> Well, my patches until this point made sure that any attempt to use a larger
> folio would fail in a way that we could spot now if there is any offender.

Ack yeah.

>
> That is why before this change, nth_page() was required within a folio.
>
> Hope that clarifies it, thanks!

Yes thanks! :)

>
> --
> Cheers
>
> David / dhildenb
>

Cheers, Lorenzo

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@nvidia.com>
>Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
>Signed-off-by: David Hildenbrand <david@redhat.com>

Reviewed-by: Wei Yang <richard.weiyang@gmail.com>

-- 
Wei Yang
Help you, Help me