Re: [PATCH v4 2/3] mm/rmap: integrate PMD-mapped folio splitting into pagewalk loop

[Lance Yang <ioworker0@gmail.com>]

On Tue, May 7, 2024 at 11:26 PM Zi Yan <ziy@nvidia.com> wrote:
>
> On 1 May 2024, at 0:26, Lance Yang wrote:
>
> > In preparation for supporting try_to_unmap_one() to unmap PMD-mapped
> > folios, start the pagewalk first, then call split_huge_pmd_address()
> > to split the folio.
> >
> > Suggested-by: David Hildenbrand <david@redhat.com>
> > Signed-off-by: Lance Yang <ioworker0@gmail.com>
> > ---
> >  include/linux/huge_mm.h | 20 ++++++++++++++++++++
> >  mm/huge_memory.c        | 42 +++++++++++++++++++++--------------------
> >  mm/rmap.c               | 24 +++++++++++++++++------
> >  3 files changed, 60 insertions(+), 26 deletions(-)
> >
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index c8d3ec116e29..38c4b5537715 100644
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -409,6 +409,20 @@ static inline bool thp_migration_supported(void)
> >       return IS_ENABLED(CONFIG_ARCH_ENABLE_THP_MIGRATION);
> >  }
> >
> > +void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address,
> > +                        pmd_t *pmd, bool freeze, struct folio *folio);
> > +
> > +static inline void align_huge_pmd_range(struct vm_area_struct *vma,
> > +                                     unsigned long *start,
> > +                                     unsigned long *end)
> > +{
> > +     *start = ALIGN(*start, HPAGE_PMD_SIZE);
> > +     *end = ALIGN_DOWN(*end, HPAGE_PMD_SIZE);
> > +
> > +     VM_WARN_ON_ONCE(vma->vm_start > *start);
> > +     VM_WARN_ON_ONCE(vma->vm_end < *end);
> > +}
> > +
> >  #else /* CONFIG_TRANSPARENT_HUGEPAGE */
> >
> >  static inline bool folio_test_pmd_mappable(struct folio *folio)
> > @@ -471,6 +485,12 @@ static inline void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
> >               unsigned long address, bool freeze, struct folio *folio) {}
> >  static inline void split_huge_pmd_address(struct vm_area_struct *vma,
> >               unsigned long address, bool freeze, struct folio *folio) {}
> > +static inline void split_huge_pmd_locked(struct vm_area_struct *vma,
> > +                                      unsigned long address, pmd_t *pmd,
> > +                                      bool freeze, struct folio *folio) {}
> > +static inline void align_huge_pmd_range(struct vm_area_struct *vma,
> > +                                     unsigned long *start,
> > +                                     unsigned long *end) {}
> >
> >  #define split_huge_pud(__vma, __pmd, __address)      \
> >       do { } while (0)
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 8261b5669397..145505a1dd05 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2584,6 +2584,27 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> >       pmd_populate(mm, pmd, pgtable);
> >  }
> >
> > +void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address,
> > +                        pmd_t *pmd, bool freeze, struct folio *folio)
> > +{
> > +     VM_WARN_ON_ONCE(folio && !folio_test_pmd_mappable(folio));
> > +     VM_WARN_ON_ONCE(!IS_ALIGNED(address, HPAGE_PMD_SIZE));
> > +     VM_WARN_ON_ONCE(folio && !folio_test_locked(folio));
> > +     VM_BUG_ON(freeze && !folio);
> > +
> > +     /*
> > +      * When the caller requests to set up a migration entry, we
> > +      * require a folio to check the PMD against. Otherwise, there
> > +      * is a risk of replacing the wrong folio.
> > +      */
> > +     if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) ||
> > +         is_pmd_migration_entry(*pmd)) {
> > +             if (folio && folio != pmd_folio(*pmd))
> > +                     return;
> > +             __split_huge_pmd_locked(vma, pmd, address, freeze);
> > +     }
> > +}
> > +
> >  void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
> >               unsigned long address, bool freeze, struct folio *folio)
> >  {
> > @@ -2595,26 +2616,7 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
> >                               (address & HPAGE_PMD_MASK) + HPAGE_PMD_SIZE);
> >       mmu_notifier_invalidate_range_start(&range);
> >       ptl = pmd_lock(vma->vm_mm, pmd);
> > -
> > -     /*
> > -      * If caller asks to setup a migration entry, we need a folio to check
> > -      * pmd against. Otherwise we can end up replacing wrong folio.
> > -      */
> > -     VM_BUG_ON(freeze && !folio);
> > -     VM_WARN_ON_ONCE(folio && !folio_test_locked(folio));
> > -
> > -     if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) ||
> > -         is_pmd_migration_entry(*pmd)) {
> > -             /*
> > -              * It's safe to call pmd_page when folio is set because it's
> > -              * guaranteed that pmd is present.
> > -              */
> > -             if (folio && folio != pmd_folio(*pmd))
> > -                     goto out;
> > -             __split_huge_pmd_locked(vma, pmd, range.start, freeze);
> > -     }
> > -
> > -out:
> > +     split_huge_pmd_locked(vma, range.start, pmd, freeze, folio);
> >       spin_unlock(ptl);
> >       mmu_notifier_invalidate_range_end(&range);
> >  }
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 7e2575d669a9..432601154583 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1636,9 +1636,6 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> >       if (flags & TTU_SYNC)
> >               pvmw.flags = PVMW_SYNC;
> >
> > -     if (flags & TTU_SPLIT_HUGE_PMD)
> > -             split_huge_pmd_address(vma, address, false, folio);
> > -
> >       /*
> >        * For THP, we have to assume the worse case ie pmd for invalidation.
> >        * For hugetlb, it could be much worse if we need to do pud
> > @@ -1650,6 +1647,8 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> >       range.end = vma_address_end(&pvmw);
> >       mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma->vm_mm,
> >                               address, range.end);
> > +     if (flags & TTU_SPLIT_HUGE_PMD)
> > +             align_huge_pmd_range(vma, &range.start, &range.end);
> >       if (folio_test_hugetlb(folio)) {
> >               /*
> >                * If sharing is possible, start and end will be adjusted
> > @@ -1664,9 +1663,6 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> >       mmu_notifier_invalidate_range_start(&range);
> >
> >       while (page_vma_mapped_walk(&pvmw)) {
> > -             /* Unexpected PMD-mapped THP? */
> > -             VM_BUG_ON_FOLIO(!pvmw.pte, folio);
> > -
> >               /*
> >                * If the folio is in an mlock()d vma, we must not swap it out.
> >                */
> > @@ -1678,6 +1674,22 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> >                       goto walk_done_err;
> >               }
> >
> > +             if (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)) {
> > +                     /*
> > +                      * We temporarily have to drop the PTL and start once
> > +                      * again from that now-PTE-mapped page table.
> > +                      */
> > +                     split_huge_pmd_locked(vma, range.start, pvmw.pmd, false,
> > +                                           folio);
>
> Just in case you might miss here, since you will no longer align
> range.start as Baolin mentioned in another email and you have a VM_WARN_ONCE
> in split_huge_pmd_locked(), you will need to align the input address now.

Thanks for bringing that up!

I do miss the alignment here when I decide to no longer align range.start
in another email - thanks!

Zi, could I move the alignment here?
IIUC, we will not encounter a partially mapped THP here, and range.start
and range.end should also not beyond the VMA limits.

                     align_huge_pmd_range(vma, &range.start, &range.end);
                     split_huge_pmd_locked(vma, range.start, pvmw.pmd, false,
                                           folio);


Thanks,
Lance

>
> > +                     pvmw.pmd = NULL;
> > +                     spin_unlock(pvmw.ptl);
> > +                     flags &= ~TTU_SPLIT_HUGE_PMD;
> > +                     continue;
> > +             }
> > +
> > +             /* Unexpected PMD-mapped THP? */
> > +             VM_BUG_ON_FOLIO(!pvmw.pte, folio);
> > +
> >               pfn = pte_pfn(ptep_get(pvmw.pte));
> >               subpage = folio_page(folio, pfn - folio_pfn(folio));
> >               address = pvmw.address;
> > --
> > 2.33.1
>
>
> --
> Best Regards,
> Yan, Zi