Re: [PATCH v6 1/2] mm/madvise: optimize lazyfreeing with mTHP in madvise_free

[Lance Yang]

Hey Ryan,

Thanks a lot for taking time to review!

On Mon, Apr 15, 2024 at 4:47 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> On 13/04/2024 01:22, Lance Yang wrote:
> > This patch optimizes lazyfreeing with PTE-mapped mTHP[1]
> > (Inspired by David Hildenbrand[2]). We aim to avoid unnecessary folio
> > splitting if the large folio is fully mapped within the target range.
> >
> > If a large folio is locked or shared, or if we fail to split it, we just
> > leave it in place and advance to the next PTE in the range. But note that
> > the behavior is changed; previously, any failure of this sort would cause
> > the entire operation to give up. As large folios become more common,
> > sticking to the old way could result in wasted opportunities.
> >
> > On an Intel I5 CPU, lazyfreeing a 1GiB VMA backed by PTE-mapped folios of
> > the same size results in the following runtimes for madvise(MADV_FREE) in
> > seconds (shorter is better):
> >
> > Folio Size |   Old    |   New    | Change
> > ------------------------------------------
> >       4KiB | 0.590251 | 0.590259 |    0%
> >      16KiB | 2.990447 | 0.185655 |  -94%
> >      32KiB | 2.547831 | 0.104870 |  -95%
> >      64KiB | 2.457796 | 0.052812 |  -97%
> >     128KiB | 2.281034 | 0.032777 |  -99%
> >     256KiB | 2.230387 | 0.017496 |  -99%
> >     512KiB | 2.189106 | 0.010781 |  -99%
> >    1024KiB | 2.183949 | 0.007753 |  -99%
> >    2048KiB | 0.002799 | 0.002804 |    0%
> >
> > [1] https://lkml.kernel.org/r/20231207161211.2374093-5-ryan.roberts@armcom
> > [2] https://lore.kernel.org/linux-mm/20240214204435.167852-1-david@xxxxxxxxxx
> >
> > Signed-off-by: Lance Yang <ioworker0@xxxxxxxxx>
>
> This is looking close IMHO. Just one bug and suggestion below.
>
>
> > ---
> >  include/linux/mm_types.h |   9 +++
> >  include/linux/pgtable.h  |  42 +++++++++++
> >  mm/internal.h            |  12 +++-
> >  mm/madvise.c             | 147 ++++++++++++++++++++++-----------------
> >  mm/memory.c              |   4 +-
> >  5 files changed, 147 insertions(+), 67 deletions(-)
> >
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index c432add95913..3c224e25f473 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -1367,6 +1367,15 @@ enum fault_flag {
> >
> >  typedef unsigned int __bitwise zap_flags_t;
> >
> > +/* Flags for clear_young_dirty_ptes(). */
> > +typedef int __bitwise cydp_t;
> > +
> > +/* make PTEs after pte_mkold() */
> > +#define CYDP_CLEAR_YOUNG             ((__force cydp_t)BIT(0))
> > +
> > +/* make PTEs after pte_mkclean() */
> > +#define CYDP_CLEAR_DIRTY             ((__force cydp_t)BIT(1))
> > +
> >  /*
> >   * FOLL_PIN and FOLL_LONGTERM may be used in various combinations with each
> >   * other. Here is what they mean, and how to use them:
> > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> > index e2f45e22a6d1..d7958243f099 100644
> > --- a/include/linux/pgtable.h
> > +++ b/include/linux/pgtable.h
> > @@ -489,6 +489,48 @@ static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
> >  }
> >  #endif
> >
> > +#ifndef clear_young_dirty_ptes
> > +/**
> > + * clear_young_dirty_ptes - Mark PTEs that map consecutive pages of the
> > + *           same folio as old/clean.
> > + * @mm: Address space the pages are mapped into.
> > + * @addr: Address the first page is mapped at.
> > + * @ptep: Page table pointer for the first entry.
> > + * @nr: Number of entries to mark old/clean.
> > + * @flags: Flags to modify the PTE batch semantics.
> > + *
> > + * May be overridden by the architecture; otherwise, implemented by
> > + * get_and_clear/modify/set for each pte in the range.
> > + *
> > + * Note that PTE bits in the PTE range besides the PFN can differ. For example,
> > + * some PTEs might be write-protected.
> > + *
> > + * Context: The caller holds the page table lock.  The PTEs map consecutive
> > + * pages that belong to the same folio.  The PTEs are all in the same PMD.
> > + */
> > +static inline void clear_young_dirty_ptes(struct mm_struct *mm,
>
> My suggestion to introduce clear_young_dirty_ptes() was so that we could
> *remove* mkold_ptes(). So I think it would be good to split that to a separate

Sorry, I forgot to remove it :(

> preparatory patch, where you change all the callers to call the new function and
> remove mkold_ptes().

Thanks for your suggestion!
I'll split this into a separate preparatory patch, modify all the callers
to use new function clear_young_dirty_ptes(), and then remove mkold_ptes().

>
> Additionally since many arches already override ptep_test_and_clear_young() (and
> that's what the default mkold_ptes() did, you might want to call that in the
> below loop if *only* CYDP_CLEAR_YOUNG is set to avoid the possibility of any
> regression. I know I only made this change for the swap-out series so what you
> have done may well be ok in practice - and certainly cleaner. It would be good
> to hear others' opinions.

It makes sense to me. I'll consider calling ptep_test_and_clear_young() in
the loop only when CYDP_CLEAR_YOUNG is set to avoid any regression.

Thanks, and I'll wait to hear others' opinions.

>
> Note the existing mkold_ptes() takes a vma instead of mm (because that's what
> ptep_test_and_clear_young() takes). So suggest passing vma. You can get mm from
> vma->mm.

Got it.

>
> > +                                       unsigned long addr, pte_t *ptep,
> > +                                       unsigned int nr, cydp_t flags)
> > +{
> > +     pte_t pte;
> > +
> > +     for (;;) {
>
> Suggestion:
>                 if (flags == CYDP_CLEAR_YOUNG) {
>                         ptep_test_and_clear_young(vma, addr, ptep);
>                 else {
>
> > +             pte = ptep_get_and_clear(mm, addr, ptep);
> > +
> > +             if (flags | CYDP_CLEAR_YOUNG)
>
> bug: this needs to be bitwise and (&). Currently it will always evaluate to
> true. Same for next one.

Sorry, my bad for the oversight and mistake :(
I'll fix it, thanks!

Thanks,
Lance

>
> > +                     pte = pte_mkold(pte);
> > +             if (flags | CYDP_CLEAR_DIRTY)
> > +                     pte = pte_mkclean(pte);
> > +
> > +             set_pte_at(mm, addr, ptep, pte);
>
>                 }
>
> > +             if (--nr == 0)
> > +                     break;
> > +             ptep++;
> > +             addr += PAGE_SIZE;
> > +     }
> > +}
> > +#endif
> > +
> >  static inline void ptep_clear(struct mm_struct *mm, unsigned long addr,
> >                             pte_t *ptep)
> >  {
> > diff --git a/mm/internal.h b/mm/internal.h
> > index 3c0f3e3f9d99..ab8fcdeaf6eb 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -134,6 +134,8 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
> >   *             first one is writable.
> >   * @any_young: Optional pointer to indicate whether any entry except the
> >   *             first one is young.
> > + * @any_dirty: Optional pointer to indicate whether any entry except the
> > + *             first one is dirty.
> >   *
> >   * Detect a PTE batch: consecutive (present) PTEs that map consecutive
> >   * pages of the same large folio.
> > @@ -149,18 +151,20 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
> >   */
> >  static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
> >               pte_t *start_ptep, pte_t pte, int max_nr, fpb_t flags,
> > -             bool *any_writable, bool *any_young)
> > +             bool *any_writable, bool *any_young, bool *any_dirty)
> >  {
> >       unsigned long folio_end_pfn = folio_pfn(folio) + folio_nr_pages(folio);
> >       const pte_t *end_ptep = start_ptep + max_nr;
> >       pte_t expected_pte, *ptep;
> > -     bool writable, young;
> > +     bool writable, young, dirty;
> >       int nr;
> >
> >       if (any_writable)
> >               *any_writable = false;
> >       if (any_young)
> >               *any_young = false;
> > +     if (any_dirty)
> > +             *any_dirty = false;
> >
> >       VM_WARN_ON_FOLIO(!pte_present(pte), folio);
> >       VM_WARN_ON_FOLIO(!folio_test_large(folio) || max_nr < 1, folio);
> > @@ -176,6 +180,8 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
> >                       writable = !!pte_write(pte);
> >               if (any_young)
> >                       young = !!pte_young(pte);
> > +             if (any_dirty)
> > +                     dirty = !!pte_dirty(pte);
> >               pte = __pte_batch_clear_ignored(pte, flags);
> >
> >               if (!pte_same(pte, expected_pte))
> > @@ -193,6 +199,8 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
> >                       *any_writable |= writable;
> >               if (any_young)
> >                       *any_young |= young;
> > +             if (any_dirty)
> > +                     *any_dirty |= dirty;
> >
> >               nr = pte_batch_hint(ptep, pte);
> >               expected_pte = pte_advance_pfn(expected_pte, nr);
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index d34ca6983227..b4103e2df346 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -321,6 +321,39 @@ static inline bool can_do_file_pageout(struct vm_area_struct *vma)
> >              file_permission(vma->vm_file, MAY_WRITE) == 0;
> >  }
> >
> > +static inline int madvise_folio_pte_batch(unsigned long addr, unsigned long end,
> > +                                       struct folio *folio, pte_t *ptep,
> > +                                       pte_t pte, bool *any_young,
> > +                                       bool *any_dirty)
> > +{
> > +     int max_nr = (end - addr) / PAGE_SIZE;
> > +     const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
> > +
> > +     return folio_pte_batch(folio, addr, ptep, pte, max_nr, fpb_flags, NULL,
> > +                            any_young, any_dirty);
> > +}
> > +
> > +static inline bool madvise_pte_split_folio(struct mm_struct *mm, pmd_t *pmd,
> > +                                        unsigned long addr,
> > +                                        struct folio *folio, pte_t **pte,
> > +                                        spinlock_t **ptl)
> > +{
> > +     int err;
> > +
> > +     if (!folio_trylock(folio))
> > +             return false;
> > +
> > +     folio_get(folio);
> > +     pte_unmap_unlock(*pte, *ptl);
> > +     err = split_folio(folio);
> > +     folio_unlock(folio);
> > +     folio_put(folio);
> > +
> > +     *pte = pte_offset_map_lock(mm, pmd, addr, ptl);
> > +
> > +     return err == 0;
> > +}
> > +
> >  static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
> >                               unsigned long addr, unsigned long end,
> >                               struct mm_walk *walk)
> > @@ -456,41 +489,30 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
> >                * next pte in the range.
> >                */
> >               if (folio_test_large(folio)) {
> > -                     const fpb_t fpb_flags = FPB_IGNORE_DIRTY |
> > -                                             FPB_IGNORE_SOFT_DIRTY;
> > -                     int max_nr = (end - addr) / PAGE_SIZE;
> >                       bool any_young;
> >
> > -                     nr = folio_pte_batch(folio, addr, pte, ptent, max_nr,
> > -                                          fpb_flags, NULL, &any_young);
> > -                     if (any_young)
> > -                             ptent = pte_mkyoung(ptent);
> > +                     nr = madvise_folio_pte_batch(addr, end, folio, pte,
> > +                                                  ptent, &any_young, NULL);
> >
> >                       if (nr < folio_nr_pages(folio)) {
> > -                             int err;
> > -
> >                               if (folio_likely_mapped_shared(folio))
> >                                       continue;
> >                               if (pageout_anon_only_filter && !folio_test_anon(folio))
> >                                       continue;
> > -                             if (!folio_trylock(folio))
> > -                                     continue;
> > -                             folio_get(folio);
> > +
> >                               arch_leave_lazy_mmu_mode();
> > -                             pte_unmap_unlock(start_pte, ptl);
> > -                             start_pte = NULL;
> > -                             err = split_folio(folio);
> > -                             folio_unlock(folio);
> > -                             folio_put(folio);
> > -                             start_pte = pte =
> > -                                     pte_offset_map_lock(mm, pmd, addr, &ptl);
> > +                             if (madvise_pte_split_folio(mm, pmd, addr,
> > +                                                         folio, &start_pte, &ptl))
> > +                                     nr = 0;
> >                               if (!start_pte)
> >                                       break;
> > +                             pte = start_pte;
> >                               arch_enter_lazy_mmu_mode();
> > -                             if (!err)
> > -                                     nr = 0;
> >                               continue;
> >                       }
> > +
> > +                     if (any_young)
> > +                             ptent = pte_mkyoung(ptent);
> >               }
> >
> >               /*
> > @@ -507,7 +529,8 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
> >                       continue;
> >
> >               if (!pageout && pte_young(ptent)) {
> > -                     mkold_ptes(vma, addr, pte, nr);
> > +                     clear_young_dirty_ptes(mm, addr, pte, nr,
> > +                                            CYDP_CLEAR_YOUNG);
> >                       tlb_remove_tlb_entries(tlb, pte, nr, addr);
> >               }
> >
> > @@ -687,44 +710,51 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> >                       continue;
> >
> >               /*
> > -              * If pmd isn't transhuge but the folio is large and
> > -              * is owned by only this process, split it and
> > -              * deactivate all pages.
> > +              * If we encounter a large folio, only split it if it is not
> > +              * fully mapped within the range we are operating on. Otherwise
> > +              * leave it as is so that it can be marked as lazyfree. If we
> > +              * fail to split a folio, leave it in place and advance to the
> > +              * next pte in the range.
> >                */
> >               if (folio_test_large(folio)) {
> > -                     int err;
> > +                     bool any_young, any_dirty;
> >
> > -                     if (folio_likely_mapped_shared(folio))
> > -                             break;
> > -                     if (!folio_trylock(folio))
> > -                             break;
> > -                     folio_get(folio);
> > -                     arch_leave_lazy_mmu_mode();
> > -                     pte_unmap_unlock(start_pte, ptl);
> > -                     start_pte = NULL;
> > -                     err = split_folio(folio);
> > -                     folio_unlock(folio);
> > -                     folio_put(folio);
> > -                     if (err)
> > -                             break;
> > -                     start_pte = pte =
> > -                             pte_offset_map_lock(mm, pmd, addr, &ptl);
> > -                     if (!start_pte)
> > -                             break;
> > -                     arch_enter_lazy_mmu_mode();
> > -                     pte--;
> > -                     addr -= PAGE_SIZE;
> > -                     continue;
> > +                     nr = madvise_folio_pte_batch(addr, end, folio, pte,
> > +                                                  ptent, &any_young, &any_dirty);
> > +
> > +                     if (nr < folio_nr_pages(folio)) {
> > +                             if (folio_likely_mapped_shared(folio))
> > +                                     continue;
> > +
> > +                             arch_leave_lazy_mmu_mode();
> > +                             if (madvise_pte_split_folio(mm, pmd, addr,
> > +                                                         folio, &start_pte, &ptl))
> > +                                     nr = 0;
> > +                             if (!start_pte)
> > +                                     break;
> > +                             pte = start_pte;
> > +                             arch_enter_lazy_mmu_mode();
> > +                             continue;
> > +                     }
> > +
> > +                     if (any_young)
> > +                             ptent = pte_mkyoung(ptent);
> > +                     if (any_dirty)
> > +                             ptent = pte_mkdirty(ptent);
> >               }
> >
> > +             if (folio_mapcount(folio) != folio_nr_pages(folio))
> > +                     continue;
> > +
> >               if (folio_test_swapcache(folio) || folio_test_dirty(folio)) {
> >                       if (!folio_trylock(folio))
> >                               continue;
> >                       /*
> > -                      * If folio is shared with others, we mustn't clear
> > -                      * the folio's dirty flag.
> > +                      * If we have a large folio at this point, we know it is
> > +                      * fully mapped so if its mapcount is the same as its
> > +                      * number of pages, it must be exclusive.
> >                        */
> > -                     if (folio_mapcount(folio) != 1) {
> > +                     if (folio_mapcount(folio) != folio_nr_pages(folio)) {
> >                               folio_unlock(folio);
> >                               continue;
> >                       }
> > @@ -740,19 +770,10 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> >               }
> >
> >               if (pte_young(ptent) || pte_dirty(ptent)) {
> > -                     /*
> > -                      * Some of architecture(ex, PPC) don't update TLB
> > -                      * with set_pte_at and tlb_remove_tlb_entry so for
> > -                      * the portability, remap the pte with old|clean
> > -                      * after pte clearing.
> > -                      */
> > -                     ptent = ptep_get_and_clear_full(mm, addr, pte,
> > -                                                     tlb->fullmm);
> > -
> > -                     ptent = pte_mkold(ptent);
> > -                     ptent = pte_mkclean(ptent);
> > -                     set_pte_at(mm, addr, pte, ptent);
> > -                     tlb_remove_tlb_entry(tlb, pte, addr);
> > +                     clear_young_dirty_ptes(mm, addr, pte, nr,
> > +                                            CYDP_CLEAR_YOUNG |
> > +                                                    CYDP_CLEAR_DIRTY);
> > +                     tlb_remove_tlb_entries(tlb, pte, nr, addr);
> >               }
> >               folio_mark_lazyfree(folio);
> >       }
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 76157b32faa8..b6fa5146b260 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -989,7 +989,7 @@ copy_present_ptes(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma
> >                       flags |= FPB_IGNORE_SOFT_DIRTY;
> >
> >               nr = folio_pte_batch(folio, addr, src_pte, pte, max_nr, flags,
> > -                                  &any_writable, NULL);
> > +                                  &any_writable, NULL, NULL);
> >               folio_ref_add(folio, nr);
> >               if (folio_test_anon(folio)) {
> >                       if (unlikely(folio_try_dup_anon_rmap_ptes(folio, page,
> > @@ -1558,7 +1558,7 @@ static inline int zap_present_ptes(struct mmu_gather *tlb,
> >        */
> >       if (unlikely(folio_test_large(folio) && max_nr != 1)) {
> >               nr = folio_pte_batch(folio, addr, pte, ptent, max_nr, fpb_flags,
> > -                                  NULL, NULL);
> > +                                  NULL, NULL, NULL);
> >
> >               zap_present_folio_ptes(tlb, vma, folio, page, pte, ptent, nr,
> >                                      addr, details, rss, force_flush,
>