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

[Lance Yang <ioworker0@gmail.com>]

On Wed, May 8, 2024 at 10:07 PM Zi Yan <ziy@nvidia.com> wrote:
>
> On 8 May 2024, at 1:43, Lance Yang wrote:
>
> > 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!
> >
> No problem.
>
> > 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);
>
> I think you can just do
>
> split_huge_pmd_locked(vma, ALIGN(range.start, HPAGE_PMD_SIZE), pvmw.pmd, false, folio);
>
> since range will later be used by mmu_notifier_invalidate_range_end() and changing
> it might cause secondary TLB invalidation issues.

Ok, makes sense to me - thanks!

But we probably cannot use the HPAGE_PMD_SIZE here; it will cause
broken compilation as seen in v3[1].

Perhaps we still need to add a new alignment function for the huge PMD?

[1] https://lore.kernel.org/linux-mm/20240429202040.187453-1-sj@kernel.org/

Thanks again for the review!

Best,
Lance



>
> >
> > 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
>
>
> --
> Best Regards,
> Yan, Zi