Re: [PATCH v4 3/5] mm/sparse-vmemmap: Fix DAX vmemmap accounting with optimization

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

> On Apr 23, 2026, at 02:53, David Hildenbrand (Arm) <david@kernel.org> wrote:
> 
> On 4/22/26 10:14, Muchun Song wrote:
>> When vmemmap optimization is enabled for DAX, the nr_memmap_pages
>> counter in /proc/vmstat is incorrect. The current code always accounts
>> for the full, non-optimized vmemmap size, but vmemmap optimization
>> reduces the actual number of vmemmap pages by reusing tail pages. This
>> causes the system to overcount vmemmap usage, leading to inaccurate
>> page statistics in /proc/vmstat.
>> 
>> Fix this by introducing section_vmemmap_pages(), which returns the exact
>> vmemmap page count for a given pfn range based on whether optimization
>> is in effect.
>> 
>> Fixes: 15995a352474 ("mm: report per-page metadata information")
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
>> Acked-by: Oscar Salvador <osalvador@suse.de>
>> ---
>> mm/sparse-vmemmap.c | 32 ++++++++++++++++++++++++++++----
>> 1 file changed, 28 insertions(+), 4 deletions(-)
>> 
>> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
>> index c208187a4b00..fcc5e0eda9e7 100644
>> --- a/mm/sparse-vmemmap.c
>> +++ b/mm/sparse-vmemmap.c
>> @@ -652,6 +652,29 @@ void offline_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
>> }
>> }
>> 
>> +static int __meminit section_vmemmap_pages(unsigned long pfn, unsigned long nr_pages,
> 
> I'd have called this "section_nr_vmemmap_pages"

No problem.

> 
>> +    struct vmem_altmap *altmap,
>> +    struct dev_pagemap *pgmap)
> 
> Two-tab indent.

OK.

> 
>> +{
>> + unsigned int order = pgmap ? pgmap->vmemmap_shift : 0;
>> + unsigned long pages_per_compound = 1L << order;
> 
> 1UL
> 
> Both can be const.

Right.

> 
>> +
>> + VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, min(pages_per_compound,
>> + PAGES_PER_SECTION)));
> 
> Maybe simply
> 
> VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, pages_per_compound));
> VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, PAGES_PER_SECTION));
> 
> Which is more readable?

That's also quite good.

> 
> 
>> + 	VM_WARN_ON_ONCE(pfn_to_section_nr(pfn) != pfn_to_section_nr(pfn + nr_pages - 1));
>> +
>> + 	if (!vmemmap_can_optimize(altmap, pgmap))
>> + 		return DIV_ROUND_UP(nr_pages * sizeof(struct page), PAGE_SIZE);
>> +
>> + 	if (order < PFN_SECTION_SHIFT)
>> + 		return VMEMMAP_RESERVE_NR * nr_pages / pages_per_compound;
>> +
>> + 	if (IS_ALIGNED(pfn, pages_per_compound))
>> + 		return VMEMMAP_RESERVE_NR;
>> +
> 
> I'll have to trust you on these ones :)

Thanks for your trust.

Thanks,
Muchun.

> 
>> + return 0;
>> +}
> 
> -- 
> Cheers,
> 
> David