Re: [PATCH 06/17] mm/sparse-vmemmap: support section-based vmemmap accounting

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

> On Jul 15, 2026, at 13:08, Mike Rapoport <rppt@kernel.org> wrote:
> 
>> section_nr_vmemmap_pages() is used to account the vmemmap pages consumed by a
>> memory section, but it currently only understands the ordinary case and the
>> pgmap-provided optimization case. That is not enough for section-based vmemmap
>> optimization, where the compound page order is carried by the memory section
>> itself and tail vmemmap pages may be shared.
>> 
>> Make the helper report the actual vmemmap footprint of a section, so it can be
>> used as the common accounting path for both ordinary and optimized sections.
>> 
>> Teach section_nr_vmemmap_pages() to use the section order when there is no
>> pgmap, and to account only the vmemmap pages that are actually needed for an
>> optimizable section. This keeps the accounting consistent with section-based
>> vmemmap optimization.
>> 
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> 
>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>> index 6fa6e7f0abf9..41a1ebbe5e85 100644
>> --- a/include/linux/mmzone.h
>> +++ b/include/linux/mmzone.h
>> @@ -2403,6 +2403,14 @@ static inline unsigned int section_order(const struct mem_section *section)
>> }
>> #endif
>> 
>> +static inline bool section_vmemmap_optimizable(const struct mem_section *section)
>> +{
>> + 	if (!is_power_of_2(sizeof(struct page)))
>> + 		return false;
>> +
>> + 	return section_order(section) >= OPTIMIZABLE_FOLIO_MIN_ORDER;
> 
> Like with page_vmemmap_optimizable() I think it's not obvious that
> section_order() is enough to gate vmemmap optimization.

page_vmemmap_optimizable() checks the optimizability of an individual page.
section_vmemmap_optimizable() checks whether the optimization can be applied
to a section. Some people may think that section_vmemmap_optimizable() checks
the optimizability of all pages within an entire section. You are referring
to that ambiguity, right?

What do you think about renaming it to section_vmemmap_optimization_eligible()?
Would that clear up the ambiguity? Or should we add a comment to
section_vmemmap_optimizableto explain its purpose?

> 
>> +}
>> +
>> /*
>>  * Fallback case for when the architecture provides its own pfn_valid() but
>>  * not a corresponding for_each_valid_pfn().
>> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
>> index 77154e9da08c..99f59394feab 100644
>> --- a/mm/sparse-vmemmap.c
>> +++ b/mm/sparse-vmemmap.c
>> @@ -645,24 +645,29 @@ 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,
>> struct vmem_altmap *altmap, struct dev_pagemap *pgmap)
>> {
>> - 	const unsigned int order = pgmap ? pgmap->vmemmap_shift : 0;
>> + 	const struct mem_section *ms = __pfn_to_section(pfn);
>> + 	const unsigned int order = pgmap ? pgmap->vmemmap_shift : section_order(ms);
>> 	const unsigned long pages_per_compound = 1UL << order;
>> + 	unsigned int vmemmap_pages = OPTIMIZED_FOLIO_VMEMMAP_PAGES;
>> 
>> 	VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, PAGES_PER_SUBSECTION));
>> 	VM_WARN_ON_ONCE(nr_pages > PAGES_PER_SECTION);
>> 
>> - 	if (!vmemmap_can_optimize(altmap, pgmap))
>> + 	if (vmemmap_can_optimize(altmap, pgmap))
>> + 		vmemmap_pages = VMEMMAP_RESERVE_NR;
> 
> I'd put it below the next if ...

Make sense. Will do.

Thanks for your review.

Muchun

> 
>> +
>> + 	if (!vmemmap_can_optimize(altmap, pgmap) && !section_vmemmap_optimizable(ms))
>> 		return DIV_ROUND_UP(nr_pages * sizeof(struct page), PAGE_SIZE);
> 
> ... and make it
> 
> vmmemap_pages = vmemmap_can_optimize() ? VMEMMAP_RESERVE_NR : 1;
> 
> -- 
> Sincerely yours,
> Mike.
>