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

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 <[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..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