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

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 <[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)."

Sounds good.

Best Regards,
Yan, Zi