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Thu, 28 Aug 2025 15:10:33 +0000 Received: from DM4PR10MB8218.namprd10.prod.outlook.com ([fe80::2650:55cf:2816:5f2]) by DM4PR10MB8218.namprd10.prod.outlook.com ([fe80::2650:55cf:2816:5f2%5]) with mapi id 15.20.9052.019; Thu, 28 Aug 2025 15:10:33 +0000 Date: Thu, 28 Aug 2025 16:10:23 +0100 From: Lorenzo Stoakes To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, Zi Yan , "Mike Rapoport (Microsoft)" , Alexander Potapenko , Andrew Morton , Brendan Jackman , Christoph Lameter , Dennis Zhou , Dmitry Vyukov , dri-devel@lists.freedesktop.org, intel-gfx@lists.freedesktop.org, iommu@lists.linux.dev, io-uring@vger.kernel.org, Jason Gunthorpe , Jens Axboe , Johannes Weiner , John Hubbard , kasan-dev@googlegroups.com, kvm@vger.kernel.org, "Liam R. Howlett" , Linus Torvalds , linux-arm-kernel@axis.com, linux-arm-kernel@lists.infradead.org, linux-crypto@vger.kernel.org, linux-ide@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mips@vger.kernel.org, linux-mmc@vger.kernel.org, linux-mm@kvack.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, linux-scsi@vger.kernel.org, Marco Elver , Marek Szyprowski , Michal Hocko , Muchun Song , netdev@vger.kernel.org, Oscar Salvador , Peter Xu , Robin Murphy , Suren Baghdasaryan , Tejun Heo , virtualization@lists.linux.dev, Vlastimil Babka , wireguard@lists.zx2c4.com, x86@kernel.org Subject: Re: [PATCH v1 11/36] mm: limit folio/compound page sizes in problematic kernel configs Message-ID: References: <20250827220141.262669-1-david@redhat.com> <20250827220141.262669-12-david@redhat.com> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250827220141.262669-12-david@redhat.com> X-ClientProxiedBy: LO4P265CA0012.GBRP265.PROD.OUTLOOK.COM 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List-Unsubscribe: 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 > Acked-by: Mike Rapoport (Microsoft) > Signed-off-by: David Hildenbrand 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. Anyway LGTM, so: Reviewed-by: Lorenzo Stoakes > --- > 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..2dee79fa2efcf 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 Hmmm, was this implicit before somehow? I mean surely by the fact as you say that physical contiguity would not otherwise be guaranteed :)) > +#else > +/* > + * There is no real limit on the folio size. We limit them to the maximum we > + * currently expect (e.g., hugetlb, dax). > + */ This is nice. > +#define MAX_FOLIO_ORDER PUD_ORDER > #endif > > #define MAX_FOLIO_NR_PAGES (1UL << MAX_FOLIO_ORDER) > -- > 2.50.1 >