Re: [PATCH v4 2/2] mm: Optimize mremap() by PTE batching

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

On Tue, Jun 10, 2025 at 3:51 PM Dev Jain <dev.jain@arm.com> wrote:
>
> Use folio_pte_batch() to optimize move_ptes(). On arm64, if the ptes
> are painted with the contig bit, then ptep_get() will iterate through all 16
> entries to collect a/d bits. Hence this optimization will result in a 16x
> reduction in the number of ptep_get() calls. Next, ptep_get_and_clear()
> will eventually call contpte_try_unfold() on every contig block, thus
> flushing the TLB for the complete large folio range. Instead, use
> get_and_clear_full_ptes() so as to elide TLBIs on each contig block, and only
> do them on the starting and ending contig block.
>
> For split folios, there will be no pte batching; nr_ptes will be 1. For
> pagetable splitting, the ptes will still point to the same large folio;
> for arm64, this results in the optimization described above, and for other
> arches (including the general case), a minor improvement is expected due to
> a reduction in the number of function calls.
>
> Signed-off-by: Dev Jain <dev.jain@arm.com>

The code appears correct to me:

Reviewed-by: Barry Song <baohua@kernel.org>

> ---
>  mm/mremap.c | 39 ++++++++++++++++++++++++++++++++-------
>  1 file changed, 32 insertions(+), 7 deletions(-)
>
> diff --git a/mm/mremap.c b/mm/mremap.c
> index 180b12225368..18b215521ada 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -170,6 +170,23 @@ static pte_t move_soft_dirty_pte(pte_t pte)
>         return pte;
>  }
>
> +static int mremap_folio_pte_batch(struct vm_area_struct *vma, unsigned long addr,
> +               pte_t *ptep, pte_t pte, int max_nr)
> +{
> +       const fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
> +       struct folio *folio;
> +
> +       if (max_nr == 1)
> +               return 1;
> +
> +       folio = vm_normal_folio(vma, addr, pte);
> +       if (!folio || !folio_test_large(folio))
> +               return 1;
> +
> +       return folio_pte_batch(folio, addr, ptep, pte, max_nr, flags, NULL,
> +                              NULL, NULL);
> +}
> +
>  static int move_ptes(struct pagetable_move_control *pmc,
>                 unsigned long extent, pmd_t *old_pmd, pmd_t *new_pmd)
>  {
> @@ -177,7 +194,7 @@ static int move_ptes(struct pagetable_move_control *pmc,
>         bool need_clear_uffd_wp = vma_has_uffd_without_event_remap(vma);
>         struct mm_struct *mm = vma->vm_mm;
>         pte_t *old_ptep, *new_ptep;
> -       pte_t pte;
> +       pte_t old_pte, pte;
>         pmd_t dummy_pmdval;
>         spinlock_t *old_ptl, *new_ptl;
>         bool force_flush = false;
> @@ -185,6 +202,8 @@ static int move_ptes(struct pagetable_move_control *pmc,
>         unsigned long new_addr = pmc->new_addr;
>         unsigned long old_end = old_addr + extent;
>         unsigned long len = old_end - old_addr;
> +       int max_nr_ptes;
> +       int nr_ptes;
>         int err = 0;
>
>         /*
> @@ -236,14 +255,16 @@ static int move_ptes(struct pagetable_move_control *pmc,
>         flush_tlb_batched_pending(vma->vm_mm);
>         arch_enter_lazy_mmu_mode();
>
> -       for (; old_addr < old_end; old_ptep++, old_addr += PAGE_SIZE,
> -                                  new_ptep++, new_addr += PAGE_SIZE) {
> +       for (; old_addr < old_end; old_ptep += nr_ptes, old_addr += nr_ptes * PAGE_SIZE,
> +               new_ptep += nr_ptes, new_addr += nr_ptes * PAGE_SIZE) {
>                 VM_WARN_ON_ONCE(!pte_none(*new_ptep));
>
> -               if (pte_none(ptep_get(old_ptep)))
> +               nr_ptes = 1;
> +               max_nr_ptes = (old_end - old_addr) >> PAGE_SHIFT;
> +               old_pte = ptep_get(old_ptep);
> +               if (pte_none(old_pte))
>                         continue;
>
> -               pte = ptep_get_and_clear(mm, old_addr, old_ptep);
>                 /*
>                  * If we are remapping a valid PTE, make sure
>                  * to flush TLB before we drop the PTL for the
> @@ -255,8 +276,12 @@ static int move_ptes(struct pagetable_move_control *pmc,
>                  * the TLB entry for the old mapping has been
>                  * flushed.
>                  */
> -               if (pte_present(pte))
> +               if (pte_present(old_pte)) {
> +                       nr_ptes = mremap_folio_pte_batch(vma, old_addr, old_ptep,
> +                                                        old_pte, max_nr_ptes);
>                         force_flush = true;
> +               }
> +               pte = get_and_clear_full_ptes(mm, old_addr, old_ptep, nr_ptes, 0);
>                 pte = move_pte(pte, old_addr, new_addr);
>                 pte = move_soft_dirty_pte(pte);
>
> @@ -269,7 +294,7 @@ static int move_ptes(struct pagetable_move_control *pmc,
>                                 else if (is_swap_pte(pte))
>                                         pte = pte_swp_clear_uffd_wp(pte);
>                         }
> -                       set_pte_at(mm, new_addr, new_ptep, pte);
> +                       set_ptes(mm, new_addr, new_ptep, pte, nr_ptes);
>                 }
>         }
>
> --
> 2.30.2
>

Thanks

Barry