On 21.08.25 22:46, Zi Yan wrote:
On 21 Aug 2025, at 16:06, 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.
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
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.
The comment about hugetlbfs is helpful here, since the other folios are still
limited by buddy allocator’s MAX_ORDER.
Yeah, but the old comment was wrong (there is DAX).
I can add here "currently expect (e.g., hugetlfs, dax)."
--
Cheers
David / dhildenb
