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

[Lorenzo Stoakes (Oracle)]

On Sat, Mar 28, 2026 at 12:45:08PM -0700, Suren Baghdasaryan wrote:
> On Fri, Mar 20, 2026 at 11:08 AM Lorenzo Stoakes (Oracle)
> <ljs@xxxxxxxxxx> 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@xxxxxxxxxx>
>
> Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

Thanks!

>
> 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.

Ah I think that's sorted out later.

>
> > +       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?

Yes, as pmd_present() will return false for a softleaf entry. This is an easy
way of figuring that out, kind of expressed a bit differently 'it's present so
maybe we have to do a TLB operation, but make sure we have a folio to operate
on'.

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

Cheers, Lorenzo