Re: [PATCH 03/49] mm/sparse: fix vmemmap page accounting for HVOed DAX

David Hildenbrand (Arm)
2026-04-13T18:39:50Z

On 4/5/26 14:51, Muchun Song wrote:
> When HVO is enabled for DAX, the vmemmap page accounting is wrong since
> it only accounts for non-HVO case.
>
> Fix the accounting by introducing section_vmemmap_pages() that returns
> the exact number of vmemmap pages needed for the given pfn range.

I guess only the accounting in /proc/meminfo is messed up, correct?

>
> Fixes: 15995a352474 ("mm: report per-page metadata information")

Similarly, please send all fixes out separately.

> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
> mm/sparse-vmemmap.c | 30 ++++++++++++++++++++++++++----
> 1 file changed, 26 insertions(+), 4 deletions(-)
>
> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> index 7aa9a97498eb..0ef96b1afbcc 100644
> --- a/mm/sparse-vmemmap.c
> +++ b/mm/sparse-vmemmap.c
> @@ -724,6 +724,27 @@ static int fill_subsection_map(unsigned long pfn, unsigned long nr_pages)
> return rc;
> }
>
> +static int __meminit section_vmemmap_pages(unsigned long pfn, unsigned long nr_pages,
> + struct vmem_altmap *altmap, struct dev_pagemap *pgmap)
> +{
> + unsigned int order = pgmap ? pgmap->vmemmap_shift : 0;
> + unsigned long pages_per_compound = 1L << order;
> +
> + VM_BUG_ON(!IS_ALIGNED(pfn | nr_pages, min(pages_per_compound, PAGES_PER_SECTION)));
> + VM_BUG_ON(pfn_to_section_nr(pfn) != pfn_to_section_nr(pfn + nr_pages - 1));

No new VM_BUG_ON. Ever. :)

VM_WARN_ON_ONCE() is your friend.

> +
> + 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;
> +
> + return 0;
> +}
> +
> /*
> * To deactivate a memory region, there are 3 cases to handle:
> *
> @@ -775,11 +796,12 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
> * section_activate() and pfn_valid() .
> */
> if (!section_is_early) {
> - memmap_pages_add(-1L * (DIV_ROUND_UP(nr_pages * sizeof(struct page), PAGE_SIZE)));

That's nice to be factored out in any case.

--
Cheers,

David