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

[Mike Rapoport <rppt@kernel.org>]

On Wed, Jul 15, 2026 at 10:44:51PM +0800, Muchun Song wrote:
> > 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?
 
The name is fine, IMHO.
What I meant is that it would be clearer if there was a check for
HUGETLB_PAGE_OPTIMIZE_VMEMMAP.
 
-- 
Sincerely yours,
Mike.