[PATCH RFC 12/35] mm: limit folio/compound page sizes in problematic kernel configs
From: David Hildenbrand <hidden>
Date: 2025-08-21 20:07:44
Also in:
dri-devel, intel-gfx, io-uring, kvm, linux-arm-kernel, linux-crypto, linux-ide, linux-iommu, linux-kselftest, linux-mips, linux-mm, linux-mmc, linux-riscv, linux-s390, linux-scsi, lkml, virtualization
Subsystem:
memory management - core, the rest · Maintainers:
Andrew Morton, David Hildenbrand, Linus Torvalds
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 <redacted> --- 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. + */ +#define MAX_FOLIO_ORDER PUD_ORDER #endif #define MAX_FOLIO_NR_PAGES (1UL << MAX_FOLIO_ORDER)
--
2.50.1