Re: [PATCH v6 4/7] mm/sparse-vmemmap: Fix DAX vmemmap accounting with optimization

[Muchun Song <muchun.song@linux.dev>]

> On Apr 25, 2026, at 13:48, David Hildenbrand (Arm) <david@kernel.org> wrote:
> 
> 
>> 
>> 
>> Hi David,
>> 
>> Sorry, I missed the 1GB hugepage scenario earlier. Given that sparse_add_section()
>> operates on a scale between PAGES_PER_SUBSECTION and PAGES_PER_SECTION, the pfn and
>> nr_pages parameters wouldn't be aligned with the hugepage size (pages_per_compound),
>> but rather with the PAGES_PER_SECTION boundary. Do you think this explanation makes
>> it clearer? In the interest of code clarity, do you think the modification below
>> makes it easier to follow?
>> 
>> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
>> index 2e642c5ff3f2..ce675c5fb94d 100644
>> --- a/mm/sparse-vmemmap.c
>> +++ b/mm/sparse-vmemmap.c
>> @@ -658,15 +658,18 @@ static int __meminit section_nr_vmemmap_pages(unsigned long pfn, unsigned long n
>>        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)));
>> +       VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, PAGES_PER_SUBSECTION));
> 
> That here makes sense. We can only add/remove in multiples of PAGES_PER_SECTION.
> I think what we are saying is that we want that check in addition to the
> existing min() check.

Right.

> 
>>        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)
>> +       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;
> 
> That makes sense as well, within a section, we expect that we always add/remove
> entire "compound"-managed chunks.
> 
>> +       }
>> +
>> +       VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, PAGES_PER_SECTION));
> 
> And this is then for the case where a 1G page spans multiple sections, where we
> expect to add/remove an entire section.
> 
> So here, indeed the "min" makes sense. I guess we also assume:
> 
>    VM_WARN_ON_ONCE(nr_pages > PAGES_PER_SECTION);

Yes. But this one we do not need to explicit it to
assert it since at the front of this function we have

VM_WARN_ON_ONCE(pfn_to_section_nr(pfn) != pfn_to_section_nr(pfn + nr_pages - 1));

to make sure the passing range belongs to one section.

Thanks.

> 
> Looks better to me!
> 
> --
> Cheers,
> 
> David