Re: [PATCH 01/11] mm/sparse-vmemmap: introduce CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION

[Muchun Song <muchun.song@linux.dev>]

> On Aug 31, 2026, at 17:44, Qi Zheng <qi.zheng@linux.dev> wrote:
> 
> 
> 
> On 8/31/26 3:53 PM, Muchun Song wrote:
>> The section-based vmemmap optimization infrastructure is still guarded by
>> CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP, but it also can be used by device
>> DAX. Introduce CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION as a common config
>> for the shared infrastructure.
>> Select the new option from HUGETLB_PAGE_OPTIMIZE_VMEMMAP and from
>> DEV_DAX when the architecture opts in to DAX vmemmap optimization, and
>> use it to guard the generic sparse-vmemmap state and helpers.
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> ---
>>  arch/x86/entry/vdso/vdso32/fake_32bit_build.h |  2 +-
>>  drivers/dax/Kconfig                           |  1 +
>>  fs/Kconfig                                    |  1 +
>>  include/linux/mm.h                            |  3 +++
>>  include/linux/mmzone.h                        | 13 +++++++------
>>  include/linux/page-flags.h                    |  5 ++---
>>  mm/Kconfig                                    |  3 +++
>>  mm/sparse.h                                   |  4 ++--
>>  8 files changed, 20 insertions(+), 12 deletions(-)
>> diff --git a/arch/x86/entry/vdso/vdso32/fake_32bit_build.h b/arch/x86/entry/vdso/vdso32/fake_32bit_build.h
>> index bc3e549795c3..5f8424eade2b 100644
>> --- a/arch/x86/entry/vdso/vdso32/fake_32bit_build.h
>> +++ b/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_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
>> +#undef CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION
>>  #undef CONFIG_NR_CPUS
>>  #undef CONFIG_PARAVIRT_XXL
>>  diff --git a/drivers/dax/Kconfig b/drivers/dax/Kconfig
>> index 602f9a0839a9..85ad4c135cdd 100644
>> --- a/drivers/dax/Kconfig
>> +++ b/drivers/dax/Kconfig
>> @@ -8,6 +8,7 @@ if DAX
>>  config DEV_DAX
>>   tristate "Device DAX: direct access mapping device"
>>   depends on TRANSPARENT_HUGEPAGE
>> + 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
>> diff --git a/fs/Kconfig b/fs/Kconfig
>> index d1c210c6508f..9b32ce79cc80 100644
>> --- a/fs/Kconfig
>> +++ b/fs/Kconfig
>> @@ -278,6 +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
>>    config HUGETLB_PMD_PAGE_TABLE_SHARING
>>   def_bool HUGETLB_PAGE
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index a9fbe26536f4..edadd7549b72 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -5188,6 +5188,9 @@ static inline bool __vmemmap_can_optimize(struct vmem_altmap *altmap,
>>   	unsigned long nr_pages;
>>   	unsigned long nr_vmemmap_pages;
>> + 	if (!IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION))
>> + 		return false;
>> +
>>   	if (!pgmap || !is_power_of_2(sizeof(struct page)))
>>   		return false;
>>  diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>> index c9ae7991a8b2..e9b54ea0eff0 100644
>> --- a/include/linux/mmzone.h
>> +++ b/include/linux/mmzone.h
>> @@ -102,9 +102,9 @@
>>   *
>>   * HVO which is only active if the size of struct page is a power of 2.
>>   */
>> -#define MAX_FOLIO_VMEMMAP_ALIGN \
>> - 	(IS_ENABLED(CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP) && \
>> -  	is_power_of_2(sizeof(struct page)) ? \
>> +#define MAX_FOLIO_VMEMMAP_ALIGN \
>> + 	(IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION) && \
>> +  	is_power_of_2(sizeof(struct page)) ? \
>>    	MAX_FOLIO_NR_PAGES * sizeof(struct page) : 0)
>>    /* The number of retained vmemmap pages with HVO enabled. */
>> @@ -116,7 +116,8 @@
>>  #define __VMEMMAP_OPTIMIZATION_NR_ORDERS \
>>   (MAX_FOLIO_ORDER - VMEMMAP_OPTIMIZATION_MIN_ORDER + 1)
>>  #define VMEMMAP_OPTIMIZATION_NR_ORDERS \
>> - 	(__VMEMMAP_OPTIMIZATION_NR_ORDERS > 0 ? __VMEMMAP_OPTIMIZATION_NR_ORDERS : 0)
>> + 	((__VMEMMAP_OPTIMIZATION_NR_ORDERS > 0 && \
>> +   	IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION)) ? __VMEMMAP_OPTIMIZATION_NR_ORDERS : 0)
>>    enum migratetype {
>>   MIGRATE_UNMOVABLE,
>> @@ -1155,7 +1156,7 @@ struct zone {
>>   	/* Zone statistics */
>>   	atomic_long_t vm_stat[NR_VM_ZONE_STAT_ITEMS];
>>   	atomic_long_t vm_numa_event[NR_VM_NUMA_EVENT_ITEMS];
>> -#ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
>> +#ifdef CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION
>>   	struct page *vmemmap_tails[VMEMMAP_OPTIMIZATION_NR_ORDERS];
>>  #endif
>>  } ____cacheline_internodealigned_in_smp;
>> @@ -2019,7 +2020,7 @@ struct mem_section {
>>   	unsigned long section_mem_map;
>>     	struct mem_section_usage *usage;
>> -#ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
>> +#ifdef CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION
>>   	/*
>>   	 * Normally, sections hold regular (order-0) pages. However, for
>>   	 * sections with HVO enabled, this tracks the compound page order
>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
>> index ae2ebaed6d4d..de3c06062bc6 100644
>> --- a/include/linux/page-flags.h
>> +++ b/include/linux/page-flags.h
>> @@ -208,14 +208,13 @@ enum pageflags {
>>  static __always_inline bool compound_info_has_mask(void)
>>  {
>>   	/*
>> - 	 * Limit mask usage to HugeTLB vmemmap optimization (HVO) where it
>> - 	 * makes a difference.
>> + 	 * Limit mask usage to HVO where it makes a difference.
>>   	 *
>>   	 * The approach with mask would work in the wider set of conditions,
>>   	 * 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_HUGETLB_PAGE_OPTIMIZE_VMEMMAP))
>> + 	if (!IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION))
>>   		return false;
>>     	return is_power_of_2(sizeof(struct page));
>> diff --git a/mm/Kconfig b/mm/Kconfig
>> index c1ddf59c0d71..b5f8372cd164 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_VMEMMAP_OPTIMIZATION
>> + bool
> 
> As sashiko was concerned about [1], it seems we need to add
> 
>        depends on SPARSEMEM_VMEMMAP
> 
> here.

You are right. Besides that, we also need DEV_DAX to depend on
ZONE_DEVICE explicitly.

> 
> Apart from that, LGTM. With this fix included:
> 
> Acked-by: Qi Zheng <qi.zheng@linux.dev>

Thanks.

> 
> Thanks,
> Qi
> 
> [1]. https://sashiko.dev/#/patchset/20260831075342.57563-1-songmuchun%40bytedance.com
> 
>> +
>>  #
>>  # Select this config option from the architecture Kconfig, if it is preferred
>>  # to enable the feature of HugeTLB/dev_dax vmemmap optimization.
>> diff --git a/mm/sparse.h b/mm/sparse.h
>> index 049272aba84e..b408d15baf7b 100644
>> --- a/mm/sparse.h
>> +++ b/mm/sparse.h
>> @@ -10,7 +10,7 @@
>>    #include <linux/mmzone.h>
>>  -#ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
>> +#ifdef CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION
>>  static inline unsigned int section_order(const struct mem_section *section)
>>  {
>>   	return section->order;
>> @@ -72,7 +72,7 @@ static inline bool vmemmap_optimizable_pfn(unsigned long pfn)
>>    static inline bool vmemmap_optimizable_order(unsigned int order)
>>  {
>> - 	if (!IS_ENABLED(CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP))
>> + 	if (!IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP_OPTIMIZATION))
>>   		return false;
>>     	if (!is_power_of_2(sizeof(struct page)))