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

[David Hildenbrand (Arm)]

On 4/25/26 08:20, Muchun Song wrote:
> 
> 
>> On Apr 25, 2026, at 13:48, David Hildenbrand (Arm) <david at 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));

Ah, yes. The alignment checks + VM_WARN_ON_ONCE(nr_pages > PAGES_PER_SECTION);
however imply that.

So you could simplify by using that check instead of the pfn_to_section_nr() check.

But it's still early here ... so whatever you prefer :)

-- 
Cheers,

David