Re: [RFC 2/2] mm/mthp: relax anon mTHP PTE Mapping restrictions

[Barry Song <21cnbao@gmail.com>]

On Mon, Jan 20, 2025 at 2:23 PM Lance Yang <ioworker0@gmail.com> wrote:
>
> Previously, mTHP could only be mapped to PTEs where all entries were none.
> With this change, PTEs within the range mapping the demand-zero page can
> now be treated as `pte_none` and remapped to a new mTHP, providing more
> opportunities to take advantage of mTHP.
>
> Signed-off-by: Mingzhe Yang <mingzhe.yang@ly.com>
> Signed-off-by: Lance Yang <ioworker0@gmail.com>
> ---
>  mm/memory.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 4e148309b3e0..99ec75c6f0fe 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4815,7 +4815,8 @@ static struct folio *alloc_anon_folio(struct vm_fault *vmf)
>         order = highest_order(orders);
>         while (orders) {
>                 addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
> -               if (pte_range_none(pte + pte_index(addr), 1 << order))
> +               if (pte_range_none_or_zeropfn(pte + pte_index(addr), 1 << order,
> +                                             NULL))
>                         break;
>                 order = next_order(&orders, order);
>         }
> @@ -4867,6 +4868,7 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>  {
>         struct vm_area_struct *vma = vmf->vma;
>         unsigned long addr = vmf->address;
> +       bool any_zeropfn = false;
>         struct folio *folio;
>         vm_fault_t ret = 0;
>         int nr_pages = 1;
> @@ -4939,7 +4941,8 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>         if (nr_pages == 1 && vmf_pte_changed(vmf)) {
>                 update_mmu_tlb(vma, addr, vmf->pte);
>                 goto release;
> -       } else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) {
> +       } else if (nr_pages > 1 && !pte_range_none_or_zeropfn(
> +                                          vmf->pte, nr_pages, &any_zeropfn)) {
>                 update_mmu_tlb_range(vma, addr, vmf->pte, nr_pages);
>                 goto release;
>         }
> @@ -4965,6 +4968,10 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>                 entry = pte_mkuffd_wp(entry);
>         set_ptes(vma->vm_mm, addr, vmf->pte, entry, nr_pages);
>
> +       /* At least one PTE was mapped to the zero page */
> +       if (nr_pages > 1 && any_zeropfn)
> +               flush_tlb_range(vma, addr, addr + (nr_pages * PAGE_SIZE));

Do we also need mmu_notifier?

        mmu_notifier_range_init(...)
        mmu_notifier_invalidate_range_start(&range);

By the way, this is getting much more complex, but are we seeing any real
benefits? I’ve tested this before, and it seems that zeropfn-mapped
anonymous folios are quite rare.

> +
>         /* No need to invalidate - it was non-present before */
>         update_mmu_cache_range(vmf, vma, addr, vmf->pte, nr_pages);
>  unlock:
> --
> 2.45.2
>

Thanks
Barry