Re: [PATCH v3 08/17] mm/sparse-vmemmap: support section-based vmemmap optimization

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

On 2026/8/4 11:55, Muchun Song 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>
> ---
> v2:
> - Keep vmemmap accounting and population logic in sparse-vmemmap.c
>    (suggested by Mike Rapoport)
> - Move vmemmap_get_tail() before its first use instead of adding only a
>    forward declaration in the previous patch (suggested by Mike Rapoport)
> - Simplify the PMD path handling for HVO-covered sections
> ---
>   mm/sparse-vmemmap.c | 36 ++++++++++++++++++++++++++++++------
>   mm/sparse.c         |  4 ++--
>   mm/sparse.h         |  7 +++++++
>   3 files changed, 39 insertions(+), 8 deletions(-)
>
> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> index b770fe2428fd..b69a7af76858 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 (pfn_vmemmap_optimizable(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);
> +
>   			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(pfn_vmemmap_optimizable(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 -ENOTSUPP;
>   			continue;
> +		}
>   		if (vmemmap_populate_basepages(addr, next, node, altmap))
>   			return -ENOMEM;
>   	}
> @@ -648,7 +672,7 @@ void offline_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
>   	}
>   }
>   
> -static int __meminit section_nr_vmemmap_pages(unsigned long pfn, unsigned long nr_pages,
> +int __meminit section_nr_vmemmap_pages(unsigned long pfn, unsigned long nr_pages,

The kernel test robot reported a compilation issue: when
CONFIG_MEMORY_HOTPLUG = n && CONFIG_SPARSEMEM_VMEMMAP=y,
section_nr_vmemmap_pages is undefined. This problem is easy to fix, and
I will move the entire function outside the CONFIG_MEMORY_HOTPLUG guard
in the next version.

Thanks.

>   		struct vmem_altmap *altmap, struct dev_pagemap *pgmap)
>   {
>   	const struct mem_section *ms = __pfn_to_section(pfn);
> diff --git a/mm/sparse.c b/mm/sparse.c
> index ca9875f568d3..24555a32a5d9 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -315,8 +315,8 @@ static void __init sparse_init_nid(int nid, unsigned long pnum_begin,
>   							nid, NULL, NULL);
>   			if (!map)
>   				panic("Failed to allocate memmap for section %lu\n", pnum);
> -			memmap_boot_pages_add(DIV_ROUND_UP(PAGES_PER_SECTION * sizeof(struct page),
> -							   PAGE_SIZE));
> +			memmap_boot_pages_add(section_nr_vmemmap_pages(pfn, PAGES_PER_SECTION,
> +								       NULL, NULL));
>   			sparse_init_early_section(nid, map, pnum, 0);
>   		}
>   	}
> diff --git a/mm/sparse.h b/mm/sparse.h
> index 6ad190ec48cf..f8f852f9f8a2 100644
> --- a/mm/sparse.h
> +++ b/mm/sparse.h
> @@ -105,8 +105,15 @@ static inline void sparse_init(void) {}
>    */
>   #ifdef CONFIG_SPARSEMEM_VMEMMAP
>   void sparse_init_subsection_map(void);
> +int __meminit section_nr_vmemmap_pages(unsigned long pfn, unsigned long nr_pages,
> +		struct vmem_altmap *altmap, struct dev_pagemap *pgmap);
>   #else
>   static inline void sparse_init_subsection_map(void) {}
> +static inline int section_nr_vmemmap_pages(unsigned long pfn, unsigned long nr_pages,
> +		struct vmem_altmap *altmap, struct dev_pagemap *pgmap)
> +{
> +	return DIV_ROUND_UP(nr_pages * sizeof(struct page), PAGE_SIZE);
> +}
>   #endif /* CONFIG_SPARSEMEM_VMEMMAP */
>   
>   #endif /* __MM_SPARSE_H */