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

[David Hildenbrand (Arm)]

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 at vger.kernel.org
> Signed-off-by: Muchun Song <songmuchun at bytedance.com>
> Acked-by: Mike Rapoport (Microsoft) <rppt at kernel.org>
> Acked-by: Oscar Salvador <osalvador at suse.de>
> ---
> 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.

Acked-by: David Hildenbrand (Arm) <david at kernel.org>

-- 
Cheers,

David