Re: [PATCH v4 3/3] mm/vmscan: avoid split lazyfree THP during shrink_folio_list()

[Lance Yang <ioworker0@gmail.com>]

Hey Zi,

Thanks for taking time to review!

On Wed, May 8, 2024 at 12:20 AM Zi Yan <ziy@nvidia.com> wrote:
>
> On 1 May 2024, at 0:27, Lance Yang wrote:
>
> > When the user no longer requires the pages, they would use
> > madvise(MADV_FREE) to mark the pages as lazy free. Subsequently, they
> > typically would not re-write to that memory again.
> >
> > During memory reclaim, if we detect that the large folio and its PMD are
> > both still marked as clean and there are no unexpected references
> > (such as GUP), so we can just discard the memory lazily, improving the
> > efficiency of memory reclamation in this case.  On an Intel i5 CPU, reclaiming 1GiB of lazyfree THPs using
> > mem_cgroup_force_empty() results in the following runtimes in seconds
> > (shorter is better):
> >
> > --------------------------------------------
> > |     Old       |      New       |  Change  |
> > --------------------------------------------
> > |   0.683426    |    0.049197    |  -92.80% |
> > --------------------------------------------
> >
> > Suggested-by: Zi Yan <ziy@nvidia.com>
> > Suggested-by: David Hildenbrand <david@redhat.com>
> > Signed-off-by: Lance Yang <ioworker0@gmail.com>
> > ---
> >  include/linux/huge_mm.h |  9 +++++
> >  mm/huge_memory.c        | 73 +++++++++++++++++++++++++++++++++++++++++
> >  mm/rmap.c               |  3 ++
> >  3 files changed, 85 insertions(+)
> >
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index 38c4b5537715..017cee864080 100644
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -411,6 +411,8 @@ static inline bool thp_migration_supported(void)
> >
> >  void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address,
> >                          pmd_t *pmd, bool freeze, struct folio *folio);
> > +bool unmap_huge_pmd_locked(struct vm_area_struct *vma, unsigned long addr,
> > +                        pmd_t *pmdp, struct folio *folio);
> >
> >  static inline void align_huge_pmd_range(struct vm_area_struct *vma,
> >                                       unsigned long *start,
> > @@ -492,6 +494,13 @@ static inline void align_huge_pmd_range(struct vm_area_struct *vma,
> >                                       unsigned long *start,
> >                                       unsigned long *end) {}
> >
> > +static inline bool unmap_huge_pmd_locked(struct vm_area_struct *vma,
> > +                                      unsigned long addr, pmd_t *pmdp,
> > +                                      struct folio *folio)
> > +{
> > +     return false;
> > +}
> > +
> >  #define split_huge_pud(__vma, __pmd, __address)      \
> >       do { } while (0)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 145505a1dd05..90fdef847a88 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2690,6 +2690,79 @@ static void unmap_folio(struct folio *folio)
> >       try_to_unmap_flush();
> >  }
> >
> > +static bool __discard_trans_pmd_locked(struct vm_area_struct *vma,
> > +                                    unsigned long addr, pmd_t *pmdp,
> > +                                    struct folio *folio)
> > +{
> > +     struct mm_struct *mm = vma->vm_mm;
> > +     int ref_count, map_count;
> > +     pmd_t orig_pmd = *pmdp;
> > +     struct mmu_gather tlb;
> > +     struct page *page;
> > +
> > +     if (pmd_dirty(orig_pmd) || folio_test_dirty(folio))
> > +             return false;
> > +     if (unlikely(!pmd_present(orig_pmd) || !pmd_trans_huge(orig_pmd)))
> > +             return false;
> > +
> > +     page = pmd_page(orig_pmd);
> > +     if (unlikely(page_folio(page) != folio))
> > +             return false;
> > +
> > +     tlb_gather_mmu(&tlb, mm);
> > +     orig_pmd = pmdp_huge_get_and_clear(mm, addr, pmdp);
> > +     tlb_remove_pmd_tlb_entry(&tlb, pmdp, addr);
> > +
> > +     /*
> > +      * Syncing against concurrent GUP-fast:
> > +      * - clear PMD; barrier; read refcount
> > +      * - inc refcount; barrier; read PMD
> > +      */
> > +     smp_mb();
> > +
> > +     ref_count = folio_ref_count(folio);
> > +     map_count = folio_mapcount(folio);
> > +
> > +     /*
> > +      * Order reads for folio refcount and dirty flag
> > +      * (see comments in __remove_mapping()).
> > +      */
> > +     smp_rmb();
> > +
> > +     /*
> > +      * If the PMD or folio is redirtied at this point, or if there are
> > +      * unexpected references, we will give up to discard this folio
> > +      * and remap it.
> > +      *
> > +      * The only folio refs must be one from isolation plus the rmap(s).
> > +      */
> > +     if (ref_count != map_count + 1 || folio_test_dirty(folio) ||
> > +         pmd_dirty(orig_pmd)) {
> > +             set_pmd_at(mm, addr, pmdp, orig_pmd);
> > +             return false;
> > +     }
> > +
> > +     folio_remove_rmap_pmd(folio, page, vma);
> > +     zap_deposited_table(mm, pmdp);
> > +     add_mm_counter(mm, MM_ANONPAGES, -HPAGE_PMD_NR);
> > +     folio_put(folio);
> > +
> > +     return true;
> > +}
> > +
> > +bool unmap_huge_pmd_locked(struct vm_area_struct *vma, unsigned long addr,
> > +                        pmd_t *pmdp, struct folio *folio)
> > +{
> > +     VM_WARN_ON_FOLIO(!folio_test_pmd_mappable(folio), folio);
> > +     VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
> > +     VM_WARN_ON_ONCE(!IS_ALIGNED(addr, HPAGE_PMD_SIZE));
> > +
> > +     if (folio_test_anon(folio) && !folio_test_swapbacked(folio))
> > +             return __discard_trans_pmd_locked(vma, addr, pmdp, folio);
> > +
> > +     return false;
> > +}
> > +
> >  static void remap_page(struct folio *folio, unsigned long nr)
> >  {
> >       int i = 0;
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 432601154583..1d3d30cb752c 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1675,6 +1675,9 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> >               }
> >
> >               if (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)) {
> > +                     if (unmap_huge_pmd_locked(vma, range.start, pvmw.pmd,
> > +                                               folio))
> > +                             goto walk_done;
>
> You might not need to check (flags & TTU_SPLIT_HUGE_PMD) for
> unmap_huge_pmd_locked(), since you are unmapping a PMD here.
> TTU_SPLIT_HUGE_PMD is here because try_to_unmap_one() was not able to unmap
> a PMD. You probably can remove it for callers that are unmapping
> the folio but not the ones are swapping.

Thanks for the suggestion!

Ageed. For unmap_huge_pmd_locked(), there is no need to check the
TTU_SPLIT_HUGE_PMD flag. We only need to check the flag for
split_huge_pmd_locked().

Given this, if we fail to remove the PMD mapping and the flag is not set,
I think we should stop the walk. So we can also remove the
VM_BUG_ON_FOLIO() below.

/* Unexpected PMD-mapped THP? */
VM_BUG_ON_FOLIO(!pvmw.pte, folio);

Zi, what do you think?

Thanks,
Lance

>
>
>
> >                       /*
> >                        * We temporarily have to drop the PTL and start once
> >                        * again from that now-PTE-mapped page table.
> > --
> > 2.33.1
>
>
> --
> Best Regards,
> Yan, Zi