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

David Hildenbrand (Arm)
2026-04-22T18:53:32Z

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"

> + struct vmem_altmap *altmap,
> + struct dev_pagemap *pgmap)

Two-tab indent.

> +{
> + unsigned int order = pgmap ? pgmap->vmemmap_shift : 0;
> + unsigned long pages_per_compound = 1L << order;

1UL

Both can be const.

> +
> + 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?

> + 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 :)

> + return 0;
> +}

--
Cheers,

David