Re: [PATCH v7 4/6] mm/sparse-vmemmap: Fix DAX vmemmap accounting with optimization

[Muchun Song]

> On Apr 27, 2026, at 18:17, David Hildenbrand (Arm) <david@xxxxxxxxxx> wrote:
> 
> On 4/26/26 11:26, 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_nr_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")
>> Cc: stable@xxxxxxxxxxxxxxx
>> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
>> Acked-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
>> Acked-by: Oscar Salvador <osalvador@xxxxxxx>
>> ---
>> v6 -> v7:
>> - Refine the alignment assertions in section_nr_vmemmap_pages().
>> ---
>> mm/sparse-vmemmap.c | 34 ++++++++++++++++++++++++++++++----
>> 1 file changed, 30 insertions(+), 4 deletions(-)
>> 
>> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
>> index 3340f6d30b01..01f448607bad 100644
>> --- a/mm/sparse-vmemmap.c
>> +++ b/mm/sparse-vmemmap.c
>> @@ -652,6 +652,31 @@ 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 unsigned long pages_per_compound = 1UL << order;
>> +
>> + 	VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, PAGES_PER_SUBSECTION));
>> +
>> + 	if (!vmemmap_can_optimize(altmap, pgmap))
>> + 		return DIV_ROUND_UP(nr_pages * sizeof(struct page), PAGE_SIZE);
>> +
>> + 	if (order < PFN_SECTION_SHIFT) {
>> + 		VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, pages_per_compound));
>> + 		return VMEMMAP_RESERVE_NR * nr_pages / pages_per_compound;
>> + 	}
>> +
>> + 	VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, PAGES_PER_SECTION));
>> + 	VM_WARN_ON_ONCE(nr_pages > PAGES_PER_SECTION);
> 
> I would just have done that at the very top, as this check applies to all cases.

My initial reasoning was that the current formula holds for compound pages smaller
than the section size, and we only need to impose limits when the page size exceeds
it. While the current callers of section_nr_vmemmap_pages() don't pass sizes larger
than a section, this will change in the future (see [1]).

I might have been overthinking the future-proofing, which led to this specific
implementation. However, I’m inclined to keep it as is for now, as I'll be updating
that series [1] soon and it will involve further changes to section_nr_vmemmap_pages().
That said, I'd love to hear your thoughts before I proceed.

[1] https://lore.kernel.org/linux-mm/20260405125240.2558577-43-songmuchun@xxxxxxxxxxxxx/


> 
> Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

Thanks.

> 
> -- 
> Cheers,
> 
> David