On 4/24/26 04:55, 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")
> Cc: [email protected]
> Signed-off-by: Muchun Song <[email protected]>
> Acked-by: Mike Rapoport (Microsoft) <[email protected]>
> Acked-by: Oscar Salvador <[email protected]>
> ---
> mm/sparse-vmemmap.c | 31 +++++++++++++++++++++++++++----
> 1 file changed, 27 insertions(+), 4 deletions(-)
>
> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> index 3340f6d30b01..2e642c5ff3f2 100644
> --- a/mm/sparse-vmemmap.c
> +++ b/mm/sparse-vmemmap.c
> @@ -652,6 +652,28 @@ 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,
> + min(pages_per_compound,
> PAGES_PER_SECTION)));
FWIW, I though the right thing to do here would be:
VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, pages_per_compound);
VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, PAGES_PER_SUBSECTION);
I don't really see how PAGES_PER_SECTION make sense given that
PAGES_PER_SUBSECTION are the smallest granularity we allow adding/removing.
Also, the "min()" implies that there is a connection between both properties,
but there isn't to that degree.
If order == 0, then you'd only ever check alignment for ... 1, not
PAGES_PER_SUBSECTION, which already looks weird.
So you really want to check "max(pages_per_compound, PAGES_PER_SUBSECTION)", but
just having two statements is clearer.
Or am I getting something very wrong here? :)
--
Cheers,
David
