[PATCH] mm: Add CONFIG_SPARSEMEM_CLASSIC

[PATCH] mm: Add CONFIG_SPARSEMEM_CLASSIC

[Kaitao Cheng]

From: Kaitao Cheng <chengkaitao@kylinos.cn>

Several MM paths identify classic sparse memory by testing
CONFIG_SPARSEMEM while also testing that CONFIG_SPARSEMEM_VMEMMAP is
disabled. Repeating the compound condition obscures the memory model being
selected and requires the local SECTION_IN_PAGE_FLAGS alias for the same
state.

Add CONFIG_SPARSEMEM_CLASSIC as a hidden derived option and use it at the
conditional sites. Remove SECTION_IN_PAGE_FLAGS and test the new option
directly.

The x86 32-bit vDSO build starts with the x86_64 configuration and then
undefines CONFIG_SPARSEMEM_VMEMMAP to emulate a 32-bit configuration.
Kconfig-derived symbols are not recomputed by the C preprocessor, so define
CONFIG_SPARSEMEM_CLASSIC there when CONFIG_SPARSEMEM is enabled. This
preserves the existing behavior.

Signed-off-by: Kaitao Cheng <chengkaitao@kylinos.cn>
---
 arch/x86/entry/vdso/vdso32/fake_32bit_build.h |  3 +++
 include/linux/mm.h                            | 12 ++++--------
 include/linux/mmzone.h                        |  2 +-
 include/linux/page-flags-layout.h             |  2 +-
 mm/Kconfig                                    |  3 +++
 mm/util.c                                     |  2 +-
 6 files changed, 13 insertions(+), 11 deletions(-)

diff --git a/arch/x86/entry/vdso/vdso32/fake_32bit_build.h b/arch/x86/entry/vdso/vdso32/fake_32bit_build.h
index 72a92cb9b53d..3e07389c4d2b 100644
--- a/arch/x86/entry/vdso/vdso32/fake_32bit_build.h
+++ b/arch/x86/entry/vdso/vdso32/fake_32bit_build.h
@@ -20,6 +20,9 @@
 #define CONFIG_PAGE_OFFSET 0
 #define CONFIG_ILLEGAL_POINTER_VALUE 0
 #define CONFIG_NR_CPUS 1
+#ifdef CONFIG_SPARSEMEM
+#define CONFIG_SPARSEMEM_CLASSIC 1
+#endif
 
 #define BUILD_VDSO32_64
 
diff --git a/include/linux/mm.h b/include/linux/mm.h
index ae56d6a2c9f9..b786ee73a20f 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -201,7 +201,7 @@ static inline void __mm_zero_struct_page(struct page *page)
 extern unsigned long sysctl_user_reserve_kbytes;
 extern unsigned long sysctl_admin_reserve_kbytes;
 
-#if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
+#ifdef CONFIG_SPARSEMEM_CLASSIC
 bool page_range_contiguous(const struct page *page, unsigned long nr_pages);
 #else
 static inline bool page_range_contiguous(const struct page *page,
@@ -2531,10 +2531,6 @@ static inline bool is_nommu_shared_vma_flags(const vma_flags_t *flags)
 }
 #endif
 
-#if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
-#define SECTION_IN_PAGE_FLAGS
-#endif
-
 /*
  * The identification function is mainly used by the buddy allocator for
  * determining if two pages could be buddies. We are not really identifying
@@ -2805,7 +2801,7 @@ static inline struct zone *folio_zone(const struct folio *folio)
 	return &folio_pgdat(folio)->node_zones[folio_zonenum(folio)];
 }
 
-#ifdef SECTION_IN_PAGE_FLAGS
+#ifdef CONFIG_SPARSEMEM_CLASSIC
 static inline void set_page_section(struct page *page, unsigned long section)
 {
 	page->flags.f &= ~(SECTIONS_MASK << SECTIONS_PGSHIFT);
@@ -2823,7 +2819,7 @@ static inline unsigned long memdesc_section(const memdesc_flags_t *mdf)
 	ASSERT_EXCLUSIVE_BITS(mdf->f, SECTIONS_MASK << SECTIONS_PGSHIFT);
 	return (mdf->f >> SECTIONS_PGSHIFT) & SECTIONS_MASK;
 }
-#else /* !SECTION_IN_PAGE_FLAGS */
+#else /* !CONFIG_SPARSEMEM_CLASSIC */
 static inline void set_page_section_from_pfn(struct page *page,
 		unsigned long pfn)
 {
@@ -2833,7 +2829,7 @@ static inline unsigned long memdesc_section(const memdesc_flags_t *mdf)
 {
 	return 0;
 }
-#endif /* SECTION_IN_PAGE_FLAGS */
+#endif /* CONFIG_SPARSEMEM_CLASSIC */
 
 /**
  * folio_pfn - Return the Page Frame Number of a folio.
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index ebbda6f31139..c83529ad6e35 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -68,7 +68,7 @@
  * memory sections).
  */
 #define MAX_FOLIO_ORDER		MAX_PAGE_ORDER
-#elif defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
+#elif defined(CONFIG_SPARSEMEM_CLASSIC)
 /*
  * Only pages within a single memory section are guaranteed to be
  * contiguous. By limiting folios to a single memory section, all folio
diff --git a/include/linux/page-flags-layout.h b/include/linux/page-flags-layout.h
index 760006b1c480..536003b364a7 100644
--- a/include/linux/page-flags-layout.h
+++ b/include/linux/page-flags-layout.h
@@ -49,7 +49,7 @@
  *      " plus space for last_cpupid: | SECTION | NODE | ZONE | LAST_CPUPID ... | FLAGS |
  * classic sparse no space for node:  | SECTION |     ZONE    | ... | FLAGS |
  */
-#if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
+#ifdef CONFIG_SPARSEMEM_CLASSIC
 #define SECTIONS_WIDTH		SECTIONS_SHIFT
 #else
 #define SECTIONS_WIDTH		0
diff --git a/mm/Kconfig b/mm/Kconfig
index 30170a936f1f..c7bbcf086f21 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -461,6 +461,9 @@ config SPARSEMEM_VMEMMAP
 	  pfn_to_page and page_to_pfn operations.  This is the most
 	  efficient option when sufficient kernel resources are available.
 
+config SPARSEMEM_CLASSIC
+	def_bool SPARSEMEM && !SPARSEMEM_VMEMMAP
+
 config VMEMMAP_OPTIMIZATION
 	bool
 	depends on SPARSEMEM_VMEMMAP
diff --git a/mm/util.c b/mm/util.c
index c5ee52aede1e..6910b47ca415 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -1590,7 +1590,7 @@ unsigned int folio_pte_batch(struct folio *folio, pte_t *ptep, pte_t pte,
 }
 #endif /* CONFIG_MMU */
 
-#if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
+#ifdef CONFIG_SPARSEMEM_CLASSIC
 /**
  * page_range_contiguous - test whether the page range is contiguous
  * @page: the start of the page range.
-- 
2.54.0 (Apple Git-157)

Re: [PATCH] mm: Add CONFIG_SPARSEMEM_CLASSIC

[Muchun Song]

> On Sep 20, 2026, at 15:04, Kaitao Cheng <kaitao.cheng@linux.dev> wrote:
> 
> From: Kaitao Cheng <chengkaitao@kylinos.cn>
> 
> Several MM paths identify classic sparse memory by testing
> CONFIG_SPARSEMEM while also testing that CONFIG_SPARSEMEM_VMEMMAP is
> disabled. Repeating the compound condition obscures the memory model being
> selected and requires the local SECTION_IN_PAGE_FLAGS alias for the same
> state.
> 
> Add CONFIG_SPARSEMEM_CLASSIC as a hidden derived option and use it at the
> conditional sites. Remove SECTION_IN_PAGE_FLAGS and test the new option
> directly.
> 
> The x86 32-bit vDSO build starts with the x86_64 configuration and then
> undefines CONFIG_SPARSEMEM_VMEMMAP to emulate a 32-bit configuration.
> Kconfig-derived symbols are not recomputed by the C preprocessor, so define
> CONFIG_SPARSEMEM_CLASSIC there when CONFIG_SPARSEMEM is enabled. This
> preserves the existing behavior.
> 
> Signed-off-by: Kaitao Cheng <chengkaitao@kylinos.cn>

Acked-by: Muchun Song <muchun.song@linux.dev>

Thanks.