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

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

> 
> 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.

>         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);

Looks better to me!

-- 
Cheers,

David