Commit 4751c39e authored by David Hildenbrand's avatar David Hildenbrand Committed by Andrew Morton
Browse files

mm: limit folio/compound page sizes in problematic kernel configs 

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.

Link: https://lkml.kernel.org/r/20250901150359.867252-12-david@redhat.com


Signed-off-by: default avatarDavid Hildenbrand <david@redhat.com>
Reviewed-by: default avatarZi Yan <ziy@nvidia.com>
Acked-by: default avatarMike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: default avatarWei Yang <richard.weiyang@gmail.com>
Reviewed-by: default avatarLorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: default avatarLiam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 99132d24

include/linux/mm.h

+18 −4
Original line number Diff line number Diff line
@@ -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 (e.g., hugetlb, dax).

 */

#define MAX_FOLIO_ORDER		PUD_ORDER

#endif



#define MAX_FOLIO_NR_PAGES	(1UL << MAX_FOLIO_ORDER)