Re: [PATCH v4 2/2] mm/madvise: optimize lazyfreeing with mTHP in madvise_free

[Lance Yang]

Hey Ryan,

Thanks for taking time to review!

On Thu, Apr 4, 2024 at 1:35 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> On 02/04/2024 13:40, 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%
>
> I'm guessing the reason that 2M is not showing any change is because its
> PMD-mapped and splitting is already elided? If you were to force it to be

Your guess is correct. The lack of change in 2M is because it's PMD-mapped.

> PTE-mapped then you'll see the very impressive speed-up there too. Don't worry
> about doing that on my account though; these results are already sufficient IMHO.
>
> >
> > [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>
> > ---
> >  mm/internal.h |  12 ++++-
> >  mm/madvise.c  | 147 ++++++++++++++++++++++++++------------------------
> >  mm/memory.c   |   4 +-
> >  3 files changed, 88 insertions(+), 75 deletions(-)
> >
> > diff --git a/mm/internal.h b/mm/internal.h
> > index 3df06a152ff0..cdc6e2162b30 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -132,6 +132,8 @@ 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.
> > @@ -147,18 +149,20 @@ 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);
> > @@ -174,6 +178,8 @@ 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))
> > @@ -191,6 +197,8 @@ 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 bd00b83e7c50..8197effd9f14 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -321,6 +321,38 @@ 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 *pte,
> > +                                       bool *any_writable, bool *any_young, bool *any_dirty)
>
> any_writable is always NULL. Do you need it?

Thanks for pointing that out.
It seems that the any_writable parameter is redundant here, so I'll drop it.

>
> > +{
> > +     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, pte, ptep_get(pte), max_nr,
>
> ptep_get() was problematic for performance of the order-0 folio case when we
> batched fork(). So we are deliberately passing around the value we already read
> in the main loop. Granted this case is not so performance critical because we
> only end up here for large folios. But I would still prefer to just pass the
> data we have already read into this function rather than reading it again.

Thanks for the explanation! I completely agree.
I‘ll pass the data we’ve already read into this function.

>
> > +                            fpb_flags, any_writable, 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,
>
> nit: I know 80 chars is a soft limit now (and I think 100 is a hard limit), but
> try to be consistent. You could move the addr param to the previous line and be
> within the 100 char limit. Personally I would just make the prototype fit in 80
> chars (same goes for madvise_folio_pte_batch).

Got it. Thanks.

>
> > +                                        spinlock_t **ptl)
> > +{
> > +     int err;
> > +
> > +     if (!folio_trylock(folio))
> > +             return false;
> > +
> > +     folio_get(folio);
> > +     pte_unmap_unlock(*pte, *ptl);
> > +     *pte = NULL;
>
> nit: you don't need this since you are later unconditionally setting it again.

Nice. I'll remove it.

>
> > +     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)
> > @@ -456,40 +488,26 @@ 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);
> > +                     nr = madvise_folio_pte_batch(addr, end, folio, pte,
> > +                                                  NULL, &any_young, NULL);
> >                       if (any_young)
> >                               ptent = pte_mkyoung(ptent);
> >
> >                       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);
> > -                             if (err)
> > -                                     continue;
> > -                             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();
> > -                             nr = 0;
> >                               continue;
>
> This change fixes a bug I've introduced in my swap-out series. Nice. I tried to
> fix in v6, but looking at this, I've realised its still broken. I've replied
> against that series with the fix.
>
>
> >                       }
> >               }
> > @@ -688,72 +706,59 @@ 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;
> > +                     nr = madvise_folio_pte_batch(addr, end, folio, pte,
> > +                                                  NULL, &any_young, &any_dirty);
> > +                     if (any_young || any_dirty)
> > +                             ptent = pte_mkdirty(pte_mkyoung(ptent));
>
> I don't think it makes any difference to how ptent is consumed below, but its
> probably more intuitive to separate these two operations:
>
>                         if (any_young)
>                                 ptent = pte_mkyoung(ptent);
>                         if (any_dirty)
>                                 ptent = pte_mkdirty(ptent);

I agree that it's more intuitive to separate these two operations. Thanks.

>
> >
> > -                     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;
> > -             }
> > +                     if (nr < folio_nr_pages(folio)) {
> > +                             if (folio_likely_mapped_shared(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 (folio_mapcount(folio) != 1) {
> > -                             folio_unlock(folio);
> > +                             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 (!folio_trylock(folio))
> > +                     continue;
> > +             /*
> > +              * 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) != folio_nr_pages(folio)) {
> > +                     folio_unlock(folio);
> > +                     continue;
> > +             }
> > +             if (folio_test_swapcache(folio) || folio_test_dirty(folio)) {
>
> I don't understand the rationale for reducing the scope of this conditional?
> Previously it was used to avoid having to lock the folio if it wasn't in the
> swapcache or if it wasn't dirty. So now you will be locking much more often.

You're right. I should keep the previous behavior of avoiding locking
the folio if
wasn't in the swapcache or if it wasn't dirty.

Thanks again for your time!
Lance

>
> Thanks,
> Ryan
>
> >                       if (folio_test_swapcache(folio) &&
> >                           !folio_free_swap(folio)) {
> >                               folio_unlock(folio);
> >                               continue;
> >                       }
> > -
> >                       folio_clear_dirty(folio);
> > -                     folio_unlock(folio);
> >               }
> > +             folio_unlock(folio);
> >
> >               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);
> > +                     mkold_clean_ptes(vma, addr, pte, nr, tlb->fullmm);
> > +                     tlb_remove_tlb_entries(tlb, pte, nr, addr);
> >               }
> >               folio_mark_lazyfree(folio);
> >       }
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 912cd738ec03..24769ecb59e5 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,
> > @@ -1559,7 +1559,7 @@ 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,
>