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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 4/28/26 04:21, Muchun Song wrote:
> 
> 
>> On Apr 27, 2026, at 18:17, David Hildenbrand (Arm) <david@kernel.org> 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: stable@vger.kernel.org
>>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>>> Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
>>> Acked-by: Oscar Salvador <osalvador@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.
> 
> 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