Re: [PATCH v4 00/11] mm: Switch device DAX to section-based vmemmap optimization

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 16 Sep 2026 14:43:30 +0800 Muchun Song <songmuchun@bytedance.com> wrote:

> This series is split out from the earlier, larger series "mm: Generalize
> HVO for HugeTLB and device DAX" [1]. While the parent series generalizes
> vmemmap optimization across HugeTLB and device DAX, this subset addresses
> a single, self-contained step: switching device DAX to the section-based
> sparse-vmemmap optimization infrastructure introduced for HugeTLB.
> 
> After the HugeTLB conversion, optimized vmemmap state is described by
> the memory section and the sparse-vmemmap population path can allocate or
> reuse shared tail vmemmap pages based on that metadata. Device DAX still
> uses the older DAX-specific population model, including a separate tail
> vmemmap page reservation and architecture-specific logic to locate or
> populate reusable tail pages.
> 
> ...
>
> This is intended to be the third smaller step toward the broader HVO
> generalization. The wider HVO consolidation between HugeTLB and device
> DAX is left for follow-up series.

Thanks, I updated mm-unstable to this version.

> v4:
> - Rename CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION to
>   CONFIG_VMEMMAP_OPTIMIZATION (suggested by Mike Rapoport)
> - Collect Acked-by tags from Mike Rapoport
> 

Here's how v4 altered mm.git:

 arch/x86/entry/vdso/vdso32/fake_32bit_build.h |    2 +-
 drivers/dax/Kconfig                           |    2 --
 fs/Kconfig                                    |    2 +-
 include/linux/mm.h                            |    2 +-
 include/linux/mmzone.h                        |    6 +++---
 include/linux/page-flags.h                    |    2 +-
 include/linux/vmemmap-optimization.h          |    6 +++---
 mm/Kconfig                                    |    3 ++-
 8 files changed, 12 insertions(+), 13 deletions(-)

--- a/arch/x86/entry/vdso/vdso32/fake_32bit_build.h~b
+++ a/arch/x86/entry/vdso/vdso32/fake_32bit_build.h
@@ -11,7 +11,7 @@
 #undef CONFIG_PGTABLE_LEVELS
 #undef CONFIG_ILLEGAL_POINTER_VALUE
 #undef CONFIG_SPARSEMEM_VMEMMAP
-#undef CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION
+#undef CONFIG_VMEMMAP_OPTIMIZATION
 #undef CONFIG_NR_CPUS
 #undef CONFIG_PARAVIRT_XXL
 
--- a/drivers/dax/Kconfig~b
+++ a/drivers/dax/Kconfig
@@ -8,8 +8,6 @@ if DAX
 config DEV_DAX
 	tristate "Device DAX: direct access mapping device"
 	depends on TRANSPARENT_HUGEPAGE
-	depends on ZONE_DEVICE
-	select SPARSEMEM_VMEMMAP_OPTIMIZATION if ARCH_WANT_OPTIMIZE_DAX_VMEMMAP
 	help
 	  Support raw access to differentiated (persistence, bandwidth,
 	  latency...) memory via an mmap(2) capable character
--- a/fs/Kconfig~b
+++ a/fs/Kconfig
@@ -278,7 +278,7 @@ config HUGETLB_PAGE_OPTIMIZE_VMEMMAP
 	def_bool HUGETLB_PAGE
 	depends on ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP
 	depends on SPARSEMEM_VMEMMAP
-	select SPARSEMEM_VMEMMAP_OPTIMIZATION
+	select VMEMMAP_OPTIMIZATION
 
 config HUGETLB_PMD_PAGE_TABLE_SHARING
 	def_bool HUGETLB_PAGE
--- a/include/linux/mm.h~b
+++ a/include/linux/mm.h
@@ -5174,7 +5174,7 @@ static inline bool __vmemmap_can_optimiz
 	unsigned long nr_pages;
 	unsigned long nr_vmemmap_pages;
 
-	if (!IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION))
+	if (!IS_ENABLED(CONFIG_VMEMMAP_OPTIMIZATION))
 		return false;
 
 	if (!pgmap || !is_power_of_2(sizeof(struct page)))
--- a/include/linux/mmzone.h~b
+++ a/include/linux/mmzone.h
@@ -103,7 +103,7 @@
  * HVO which is only active if the size of struct page is a power of 2.
  */
 #define MAX_FOLIO_VMEMMAP_ALIGN					\
-	(IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION) &&	\
+	(IS_ENABLED(CONFIG_VMEMMAP_OPTIMIZATION) &&		\
 	 is_power_of_2(sizeof(struct page)) ?			\
 	 MAX_FOLIO_NR_PAGES * sizeof(struct page) : 0)
 
@@ -117,7 +117,7 @@
 	(MAX_FOLIO_ORDER - VMEMMAP_OPTIMIZATION_MIN_ORDER + 1)
 #define VMEMMAP_OPTIMIZATION_NR_ORDERS		\
 	((__VMEMMAP_OPTIMIZATION_NR_ORDERS > 0 &&	\
-	  IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION)) ? __VMEMMAP_OPTIMIZATION_NR_ORDERS : 0)
+	  IS_ENABLED(CONFIG_VMEMMAP_OPTIMIZATION)) ? __VMEMMAP_OPTIMIZATION_NR_ORDERS : 0)
 
 enum migratetype {
 	MIGRATE_UNMOVABLE,
@@ -2020,7 +2020,7 @@ struct mem_section {
 	unsigned long section_mem_map;
 
 	struct mem_section_usage *usage;
-#ifdef CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION
+#ifdef CONFIG_VMEMMAP_OPTIMIZATION
 	/*
 	 * Normally, sections hold regular (order-0) pages. However, for
 	 * sections with HVO enabled, this tracks the compound page order
--- a/include/linux/page-flags.h~b
+++ a/include/linux/page-flags.h
@@ -214,7 +214,7 @@ static __always_inline bool compound_inf
 	 * but it requires validating that struct pages are naturally aligned
 	 * for all orders up to the MAX_FOLIO_ORDER, which can be tricky.
 	 */
-	if (!IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION))
+	if (!IS_ENABLED(CONFIG_VMEMMAP_OPTIMIZATION))
 		return false;
 
 	return is_power_of_2(sizeof(struct page));
--- a/include/linux/vmemmap-optimization.h~b
+++ a/include/linux/vmemmap-optimization.h
@@ -14,7 +14,7 @@
 #include <linux/mmdebug.h>
 #include <linux/mmzone.h>
 
-#ifdef CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION
+#ifdef CONFIG_VMEMMAP_OPTIMIZATION
 static inline unsigned int section_compound_order(const struct mem_section *section)
 {
 	return section->compound_page_order;
@@ -64,7 +64,7 @@ static inline unsigned int pfn_to_sectio
 {
 	return 0;
 }
-#endif /* CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION */
+#endif /* CONFIG_VMEMMAP_OPTIMIZATION */
 
 static inline bool vmemmap_optimizable_pfn(unsigned long pfn)
 {
@@ -79,7 +79,7 @@ static inline bool vmemmap_optimizable_p
 
 static inline bool vmemmap_optimizable_order(unsigned int order)
 {
-	if (!IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION))
+	if (!IS_ENABLED(CONFIG_VMEMMAP_OPTIMIZATION))
 		return false;
 
 	if (!is_power_of_2(sizeof(struct page)))
--- a/mm/Kconfig~b
+++ a/mm/Kconfig
@@ -461,7 +461,7 @@ 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_VMEMMAP_OPTIMIZATION
+config VMEMMAP_OPTIMIZATION
 	bool
 	depends on SPARSEMEM_VMEMMAP
 
@@ -1224,6 +1224,7 @@ config ZONE_DMA32
 config ZONE_DEVICE
 	bool "Device memory (pmem, HMM, etc...) hotplug support"
 	depends on MEMORY_HOTREMOVE
+	select VMEMMAP_OPTIMIZATION if ARCH_WANT_OPTIMIZE_DAX_VMEMMAP
 	select XARRAY_MULTI
 
 	help
_