linux-kernel - Re: [PATCH v1 11/36] mm: limit folio/compound page sizes in problematic kernel configs

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Message-ID: <eff8badd-0ddd-4a5f-a2ef-0e3ded39687a@redhat.com>
Date: Fri, 29 Aug 2025 13:57:22 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: linux-kernel@...r.kernel.org, Zi Yan <ziy@...dia.com>,
 "Mike Rapoport (Microsoft)" <rppt@...nel.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,
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 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,
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 Vlastimil Babka <vbabka@...e.cz>, wireguard@...ts.zx2c4.com, x86@...nel.org
Subject: Re: [PATCH v1 11/36] mm: limit folio/compound page sizes in
 problematic kernel configs

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@...dia.com>
>> Acked-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
>> Signed-off-by: David Hildenbrand <david@...hat.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