Re: [PATCH v4 08/17] mm/sparse-vmemmap: support section-based vmemmap optimization

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

On 2026/8/24 16:59, Mike Rapoport wrote:
>> Teach sparse-vmemmap population code to use the compound page order
>> when deciding whether a vmemmap page can be optimized.
>>
>> With this information, the common sparse-vmemmap population path can
>> allocate or reuse shared tail vmemmap pages directly instead of relying
>> on HugeTLB-specific handling.
>>
>> This centralizes vmemmap optimization logic in the sparse-vmemmap code,
>> based on section metadata, and prepares for sharing the same mechanism
>> across different users of vmemmap optimization, including HugeTLB and
>> DAX.
>>
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>>
>> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
>> index b770fe2428fd..737d50bdd3ef 100644
>> --- a/mm/sparse-vmemmap.c
>> +++ b/mm/sparse-vmemmap.c
>> @@ -186,6 +186,11 @@ static __meminit struct page *vmemmap_get_tail(unsigned int order, struct zone *
>>   
>>   	return tail;
>>   }
>> +#else
>> +static inline struct page *vmemmap_get_tail(unsigned int order, struct zone *zone)
>> +{
>> +	return NULL;
>> +}
>>   #endif
>>   
>>   static pte_t * __meminit vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, int node,
>> @@ -193,12 +198,24 @@ static pte_t * __meminit vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, in
>>   				       unsigned long ptpfn, unsigned long flags)
>>   {
>>   	pte_t *pte = pte_offset_kernel(pmd, addr);
>> +	unsigned long pfn = page_to_pfn((struct page *)addr);
>> +
>>   	if (pte_none(ptep_get(pte))) {
>>   		pte_t entry;
>> -		void *p;
>> +
>> +		if (vmemmap_optimizable_pfn(pfn) && ptpfn == (unsigned long)-1) {
>> +			unsigned int order = pfn_to_section_order(pfn);
>> +			struct zone *zone = pfn_to_zone(pfn, node);
>> +			struct page *page = vmemmap_get_tail(order, zone);
>> +
>> +			if (!page)
>> +				return NULL;
>> +			ptpfn = page_to_pfn(page);
>> +		}
>>   
>>   		if (ptpfn == (unsigned long)-1) {
>> -			p = vmemmap_alloc_block_buf(PAGE_SIZE, node, altmap);
>> +			void *p = vmemmap_alloc_block_buf(PAGE_SIZE, node, altmap);
>> +
> What do you think about adding a helper function here, e.g something
> like

Better than mine. More clear.

>
> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> index 0aa0ab4f8b11..933b122f9937 100644
> --- a/mm/sparse-vmemmap.c
> +++ b/mm/sparse-vmemmap.c
> @@ -191,6 +191,23 @@ static inline struct page *vmemmap_get_tail(unsigned int order, struct zone *zon
>   }
>   #endif
>   
> +static void * __meminit vmemmap_alloc_pte(unsigned long pfn, int node,
> +					  struct vmem_altmap *altmap)
> +{
> +	unsigned int order = pfn_to_section_order(pfn);
> +	struct zone *zone = pfn_to_zone(pfn, node);
> +	struct page *page;
> +
> +	if (!vmemmap_optimizable_pfn(pfn))
> +		return vmemmap_alloc_block_buf(PAGE_SIZE, node, altmap);
> +
> +	page = vmemmap_get_tail(order, zone);
> +	if (!page)
> +		return NULL;
> +
> +	return page_address(page);
> +}
> +
>   static pte_t * __meminit vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, int node,
>   				       struct vmem_altmap *altmap,
>   				       unsigned long ptpfn, unsigned long flags)
> @@ -201,19 +218,8 @@ static pte_t * __meminit vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, in
>   	if (pte_none(ptep_get(pte))) {
>   		pte_t entry;
>   
> -		if (vmemmap_optimizable_pfn(pfn) && ptpfn == (unsigned long)-1) {
> -			unsigned int order = pfn_to_section_order(pfn);
> -			struct zone *zone = pfn_to_zone(pfn, node);
> -			struct page *page = vmemmap_get_tail(order, zone);
> -
> -			if (!page)
> -				return NULL;
> -			ptpfn = page_to_pfn(page);
> -		}
> -
>   		if (ptpfn == (unsigned long)-1) {
> -			void *p = vmemmap_alloc_block_buf(PAGE_SIZE, node, altmap);
> -
> +			void *p = vmemmap_alloc_pte(pfn, node, altmap);
>   			if (!p)
>   				return NULL;
>   			ptpfn = PHYS_PFN(__pa(p));
>
>>   			if (!p)
>>   				return NULL;
>>   			ptpfn = PHYS_PFN(__pa(p));
>> @@ -217,7 +234,8 @@ static pte_t * __meminit vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, in
>>   		}
>>   		entry = pfn_pte(ptpfn, PAGE_KERNEL);
>>   		set_pte_at(&init_mm, addr, pte, entry);
>> -	}
>> +	} else if (WARN_ON_ONCE(vmemmap_optimizable_pfn(pfn)))
>> +		return NULL;
>>   	return pte;
>>   }
>>   
>> @@ -406,6 +424,9 @@ int __meminit vmemmap_populate_hugepages(unsigned long start, unsigned long end,
>>   	pmd_t *pmd;
>>   
>>   	for (addr = start; addr < end; addr = next) {
>> +		unsigned long pfn = page_to_pfn((struct page *)addr);
>> +		const struct mem_section *ms = __pfn_to_section(pfn);
>> +
>>   		next = pmd_addr_end(addr, end);
>>   
>>   		pgd = vmemmap_pgd_populate(addr, node);
>> @@ -421,7 +442,7 @@ int __meminit vmemmap_populate_hugepages(unsigned long start, unsigned long end,
>>   			return -ENOMEM;
>>   
>>   		pmd = pmd_offset(pud, addr);
>> -		if (pmd_none(pmdp_get(pmd))) {
>> +		if (pmd_none(pmdp_get(pmd)) && !section_vmemmap_optimizable(ms)) {
>>   			void *p;
>>   
>>   			p = vmemmap_alloc_block_buf(PMD_SIZE, node, altmap);
>> @@ -439,8 +460,11 @@ int __meminit vmemmap_populate_hugepages(unsigned long start, unsigned long end,
>>   				 */
>>   				return -ENOMEM;
>>   			}
>> -		} else if (vmemmap_check_pmd(pmd, node, addr, next))
>> +		} else if (vmemmap_check_pmd(pmd, node, addr, next)) {
>> +			if (WARN_ON_ONCE(section_vmemmap_optimizable(ms)))
>> +				return -EOPNOTSUPP;
>>   			continue;
>> +		}
>>   		if (vmemmap_populate_basepages(addr, next, node, altmap))
>>   			return -ENOMEM;
>>   	}
>> @@ -620,6 +644,33 @@ void __init sparse_init_subsection_map(void)
>>   		sparse_init_subsection_map_range(start, end - start);
>>   }
>>   
>> +int __meminit section_nr_vmemmap_pages(unsigned long pfn, unsigned long nr_pages,
>> +		struct vmem_altmap *altmap, struct dev_pagemap *pgmap)
>> +{
> Can we do all code movements in separate patches?

Yes.

> Maybe even move this to the previous patch and update the changelog to
> say "move code around in preparation ..."

Make sense. Will do next version.

Muchun,
Thanks.