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

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Message-ID: <48255144-572E-4BB9-BBA0-D446DCBA8D75@nvidia.com>
Date: Thu, 21 Aug 2025 16:50:03 -0400
From: Zi Yan <ziy@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.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, intel-gfx@...ts.freedesktop.org,
 iommu@...ts.linux.dev, io-uring@...r.kernel.org,
 Jason Gunthorpe <jgg@...dia.com>, Jens Axboe <axboe@...nel.dk>,
 Johannes Weiner <hannes@...xchg.org>, John Hubbard <jhubbard@...dia.com>,
 kasan-dev@...glegroups.com, kvm@...r.kernel.org,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>, linux-arm-kernel@...s.com,
 linux-arm-kernel@...ts.infradead.org, linux-crypto@...r.kernel.org,
 linux-ide@...r.kernel.org, linux-kselftest@...r.kernel.org,
 linux-mips@...r.kernel.org, linux-mmc@...r.kernel.org, linux-mm@...ck.org,
 linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
 linux-scsi@...r.kernel.org, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Marco Elver <elver@...gle.com>, Marek Szyprowski <m.szyprowski@...sung.com>,
 Michal Hocko <mhocko@...e.com>, Mike Rapoport <rppt@...nel.org>,
 Muchun Song <muchun.song@...ux.dev>, netdev@...r.kernel.org,
 Oscar Salvador <osalvador@...e.de>, Peter Xu <peterx@...hat.com>,
 Robin Murphy <robin.murphy@....com>, Suren Baghdasaryan <surenb@...gle.com>,
 Tejun Heo <tj@...nel.org>, virtualization@...ts.linux.dev,
 Vlastimil Babka <vbabka@...e.cz>, wireguard@...ts.zx2c4.com, x86@...nel.org
Subject: 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 <david@...hat.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