Re: [PATCH v3 3/3] mm/vmscan: avoid split lazyfree THP during shrink_folio_list()

[Lance Yang]

Hey Barry,

Thanks for taking time to review!

On Tue, Apr 30, 2024 at 4:35 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> On Tue, Apr 30, 2024 at 1:23 AM Lance Yang <ioworker0@xxxxxxxxx> 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@xxxxxxxxxx>
> > Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> > Signed-off-by: Lance Yang <ioworker0@xxxxxxxxx>
> > ---
> >  include/linux/huge_mm.h |  2 ++
> >  mm/huge_memory.c        | 75 +++++++++++++++++++++++++++++++++++++++++
> >  mm/rmap.c               |  3 ++
> >  3 files changed, 80 insertions(+)
> >
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index 2daadfcc6776..fd330f72b4f3 100644
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -38,6 +38,8 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
> >                     unsigned long cp_flags);
> >  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);
> >
> >  vm_fault_t vmf_insert_pfn_pmd(struct vm_fault *vmf, pfn_t pfn, bool write);
> >  vm_fault_t vmf_insert_pfn_pud(struct vm_fault *vmf, pfn_t pfn, bool write);
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 145505a1dd05..d35d526ed48f 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2690,6 +2690,81 @@ 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));
> > +
> > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > +       if (folio_test_anon(folio) && !folio_test_swapbacked(folio))
> > +               return __discard_trans_pmd_locked(vma, addr, pmdp, folio);
> > +#endif
>
> this is weird and huge_memory.c is only built with
> CONFIG_TRANSPARENT_HUGEPAGE = y;
>
> mm/Makefile:
> obj-$(CONFIG_TRANSPARENT_HUGEPAGE) += huge_memory.o khugepaged.o

Thanks for pointing that out!
I'll drop the conditional compilation directives :)

>
> > +
> > +       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 e42f436c7ff3..ab37af4f47aa 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1677,6 +1677,9 @@ 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;
>
> this is making
> mm/rmap.c:1680: undefined reference to `unmap_huge_pmd_locked'
> mm/rmap.c:1687: undefined reference to `split_huge_pmd_locked'

You're right!

It's my oversight, and I'll make sure to address it in the next version.

Thanks again for the review!
Lance

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