[v1] mm: remove nth_page()

[PATCH RFC 12/35] mm: limit folio/compound page sizes in problematic kernel configs

Posted by David Hildenbrand 1 month, 1 week ago

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.

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..48a985e17ef4e 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.
+ */
+#define MAX_FOLIO_ORDER		PUD_ORDER
 #endif
 
 #define MAX_FOLIO_NR_PAGES	(1UL << MAX_FOLIO_ORDER)
-- 
2.50.1

Re: [PATCH RFC 12/35] mm: limit folio/compound page sizes in problematic kernel configs

Posted by Mike Rapoport 1 month, 1 week ago

On Thu, Aug 21, 2025 at 10:06:38PM +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.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>

Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
>  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..48a985e17ef4e 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.
> + */
> +#define MAX_FOLIO_ORDER		PUD_ORDER
>  #endif
>  
>  #define MAX_FOLIO_NR_PAGES	(1UL << MAX_FOLIO_ORDER)
> -- 
> 2.50.1
> 

-- 
Sincerely yours,
Mike.

Re: [PATCH RFC 12/35] mm: limit folio/compound page sizes in problematic kernel configs

Posted by Zi Yan 1 month, 1 week ago

On 21 Aug 2025, at 16:06, 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.
>
> 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..48a985e17ef4e 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.

The comment about hugetlbfs is helpful here, since the other folios are still
limited by buddy allocator’s MAX_ORDER.

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

Otherwise, Reviewed-by: Zi Yan <ziy@nvidia.com>

Best Regards,
Yan, Zi

Re: [PATCH RFC 12/35] mm: limit folio/compound page sizes in problematic kernel configs

Posted by David Hildenbrand 1 month, 1 week ago

On 21.08.25 22:46, Zi Yan wrote:
> On 21 Aug 2025, at 16:06, 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.
>>
>> 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..48a985e17ef4e 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.
> 
> The comment about hugetlbfs is helpful here, since the other folios are still
> limited by buddy allocator’s MAX_ORDER.

Yeah, but the old comment was wrong (there is DAX).

I can add here "currently expect (e.g., hugetlfs, dax)."

-- 
Cheers

David / dhildenb

Re: [PATCH RFC 12/35] mm: limit folio/compound page sizes in problematic kernel configs

Posted by Zi Yan 1 month, 1 week ago

On 21 Aug 2025, at 16:49, David Hildenbrand wrote:

> On 21.08.25 22:46, Zi Yan wrote:
>> On 21 Aug 2025, at 16:06, 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.
>>>
>>> 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..48a985e17ef4e 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.
>>
>> The comment about hugetlbfs is helpful here, since the other folios are still
>> limited by buddy allocator’s MAX_ORDER.
>
> Yeah, but the old comment was wrong (there is DAX).
>
> I can add here "currently expect (e.g., hugetlfs, dax)."

Sounds good.

Best Regards,
Yan, Zi