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@nvidia.com>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Wei Yang <redacted>
Reviewed-by: Lorenzo Stoakes <redacted>
Reviewed-by: Liam R. Howlett <redacted>
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..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
+#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)

-- 
2.50.1