On 4/28/26 04:21, Muchun Song wrote: > > >> On Apr 27, 2026, at 18:17, David Hildenbrand (Arm) <[email protected]> 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: [email protected] >>> Signed-off-by: Muchun Song <[email protected]> >>> Acked-by: Mike Rapoport (Microsoft) <[email protected]> >>> Acked-by: Oscar Salvador <[email protected]> >>> --- >>> 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]).
A function that is called *section_* will get a range that exceeds a section? That sounds conceptually wrong, no? -- Cheers, David
