Re: [PATCH v3 12/13] mm/huge_memory: add and use normal_or_softleaf_folio_pmd()

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Mar 20, 2026 at 11:08 AM Lorenzo Stoakes (Oracle)
<ljs@kernel.org> wrote:
>
> Now we have pmd_to_softleaf_folio() available to us which also raises a
> CONFIG_DEBUG_VM warning if unexpectedly an invalid softleaf entry, we can
> now abstract folio handling altogether.
>
> vm_normal_folio() deals with the huge zero page (which is present), as well
> as PFN map/mixed map mappings in both cases returning NULL.
>
> Otherwise, we try to obtain the softleaf folio.
>
> This makes the logic far easier to comprehend and has it use the standard
> vm_normal_folio_pmd() path for decoding of present entries.
>
> Finally, we have to update the flushing logic to only do so if a folio is
> established.
>
> This patch also makes the 'is_present' value more accurate - because PFN
> map, mixed map and zero huge pages are present, just not present and
> 'normal'.
>
> Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>

Reviewed-by: Suren Baghdasaryan <surenb@google.com>

One clarifying question below but I think I know the answer, just want
to double-check.

> ---
>  mm/huge_memory.c | 47 +++++++++++++++++++----------------------------
>  1 file changed, 19 insertions(+), 28 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 9ddf38d68406..5831966391bd 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2342,10 +2342,6 @@ static void zap_huge_pmd_folio(struct mm_struct *mm, struct vm_area_struct *vma,
>                 add_mm_counter(mm, mm_counter_file(folio),
>                                -HPAGE_PMD_NR);
>
> -               /*
> -                * Use flush_needed to indicate whether the PMD entry
> -                * is present, instead of checking pmd_present() again.
> -                */
>                 if (is_present && pmd_young(pmdval) &&
>                     likely(vma_has_recency(vma)))
>                         folio_mark_accessed(folio);
> @@ -2356,6 +2352,17 @@ static void zap_huge_pmd_folio(struct mm_struct *mm, struct vm_area_struct *vma,
>                 folio_put(folio);
>  }
>
> +static struct folio *normal_or_softleaf_folio_pmd(struct vm_area_struct *vma,
> +               unsigned long addr, pmd_t pmdval, bool is_present)
> +{
> +       if (is_present)
> +               return vm_normal_folio_pmd(vma, addr, pmdval);
> +
> +       if (!thp_migration_supported())
> +               WARN_ONCE(1, "Non present huge pmd without pmd migration enabled!");
> +       return pmd_to_softleaf_folio(pmdval);
> +}
> +
>  /**
>   * zap_huge_pmd - Zap a huge THP which is of PMD size.
>   * @tlb: The MMU gather TLB state associated with the operation.
> @@ -2390,36 +2397,20 @@ bool zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>                                                 tlb->fullmm);
>         arch_check_zapped_pmd(vma, orig_pmd);
>         tlb_remove_pmd_tlb_entry(tlb, pmd, addr);
> -       if (vma_is_special_huge(vma))
> -               goto out;
> -       if (is_huge_zero_pmd(orig_pmd)) {
> -               if (!vma_is_dax(vma))
> -                       has_deposit = true;
> -               goto out;
> -       }
>
> -       if (pmd_present(orig_pmd)) {
> -               folio = pmd_folio(orig_pmd);
> -               is_present = true;
> -       } else if (pmd_is_valid_softleaf(orig_pmd)) {
> -               const softleaf_t entry = softleaf_from_pmd(orig_pmd);
> +       is_present = pmd_present(orig_pmd);

nit: With this you don't need to initialize is_present anymore when
you define it.

> +       folio = normal_or_softleaf_folio_pmd(vma, addr, orig_pmd, is_present);
> +       if (folio)
> +               zap_huge_pmd_folio(mm, vma, orig_pmd, folio, is_present,
> +                                  &has_deposit);
> +       else if (is_huge_zero_pmd(orig_pmd))
> +               has_deposit = !vma_is_dax(vma);
>
> -               folio = softleaf_to_folio(entry);
> -               if (!thp_migration_supported())
> -                       WARN_ONCE(1, "Non present huge pmd without pmd migration enabled!");
> -       } else {
> -               WARN_ON_ONCE(true);
> -               goto out;
> -       }
> -
> -       zap_huge_pmd_folio(mm, vma, orig_pmd, folio, is_present, &has_deposit);
> -
> -out:
>         if (has_deposit)
>                 zap_deposited_table(mm, pmd);
>
>         spin_unlock(ptl);
> -       if (is_present)
> +       if (is_present && folio)

In which case would you have a valid folio and !is_present? Is that
the softleaf case?

>                 tlb_remove_page_size(tlb, &folio->page, HPAGE_PMD_SIZE);
>         return true;
>  }
> --
> 2.53.0
>