Re: [PATCH v4 6/6] mm: madvise: Avoid split during MADV_PAGEOUT and MADV_COLD

[Barry Song <21cnbao@gmail.com>]

On Wed, Mar 13, 2024 at 10:03 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> On 13/03/2024 07:19, Barry Song wrote:
> > On Tue, Mar 12, 2024 at 4:01 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
> >>
> >> Rework madvise_cold_or_pageout_pte_range() to avoid splitting any large
> >> folio that is fully and contiguously mapped in the pageout/cold vm
> >> range. This change means that large folios will be maintained all the
> >> way to swap storage. This both improves performance during swap-out, by
> >> eliding the cost of splitting the folio, and sets us up nicely for
> >> maintaining the large folio when it is swapped back in (to be covered in
> >> a separate series).
> >>
> >> Folios that are not fully mapped in the target range are still split,
> >> but note that behavior is changed so that if the split fails for any
> >> reason (folio locked, shared, etc) we now leave it as is and move to the
> >> next pte in the range and continue work on the proceeding folios.
> >> Previously any failure of this sort would cause the entire operation to
> >> give up and no folios mapped at higher addresses were paged out or made
> >> cold. Given large folios are becoming more common, this old behavior
> >> would have likely lead to wasted opportunities.
> >>
> >> While we are at it, change the code that clears young from the ptes to
> >> use ptep_test_and_clear_young(), which is more efficent than
> >> get_and_clear/modify/set, especially for contpte mappings on arm64,
> >> where the old approach would require unfolding/refolding and the new
> >> approach can be done in place.
> >>
> >> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> >
> > This looks so much better than our initial RFC.
> > Thank you for your excellent work!
>
> Thanks - its a team effort - I had your PoC and David's previous batching work
> to use as a template.
>
> >
> >> ---
> >>  mm/madvise.c | 89 ++++++++++++++++++++++++++++++----------------------
> >>  1 file changed, 51 insertions(+), 38 deletions(-)
> >>
> >> diff --git a/mm/madvise.c b/mm/madvise.c
> >> index 547dcd1f7a39..56c7ba7bd558 100644
> >> --- a/mm/madvise.c
> >> +++ b/mm/madvise.c
> >> @@ -336,6 +336,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
> >>         LIST_HEAD(folio_list);
> >>         bool pageout_anon_only_filter;
> >>         unsigned int batch_count = 0;
> >> +       int nr;
> >>
> >>         if (fatal_signal_pending(current))
> >>                 return -EINTR;
> >> @@ -423,7 +424,8 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
> >>                 return 0;
> >>         flush_tlb_batched_pending(mm);
> >>         arch_enter_lazy_mmu_mode();
> >> -       for (; addr < end; pte++, addr += PAGE_SIZE) {
> >> +       for (; addr < end; pte += nr, addr += nr * PAGE_SIZE) {
> >> +               nr = 1;
> >>                 ptent = ptep_get(pte);
> >>
> >>                 if (++batch_count == SWAP_CLUSTER_MAX) {
> >> @@ -447,55 +449,66 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
> >>                         continue;
> >>
> >>                 /*
> >> -                * Creating a THP page is expensive so split it only if we
> >> -                * are sure it's worth. Split it if we are only owner.
> >> +                * 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 swapped out whole. 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;
> >> -
> >> -                       if (folio_estimated_sharers(folio) > 1)
> >> -                               break;
> >> -                       if (pageout_anon_only_filter && !folio_test_anon(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;
> >> +                       const fpb_t fpb_flags = FPB_IGNORE_DIRTY |
> >> +                                               FPB_IGNORE_SOFT_DIRTY;
> >> +                       int max_nr = (end - addr) / PAGE_SIZE;
> >> +
> >> +                       nr = folio_pte_batch(folio, addr, pte, ptent, max_nr,
> >> +                                            fpb_flags, NULL);
> >
> > I wonder if we have a quick way to avoid folio_pte_batch() if users
> > are doing madvise() on a portion of a large folio.
>
> Good idea. Something like this?:
>
>         if (pte_pfn(pte) == folio_pfn(folio)

what about

"If (pte_pfn(pte) == folio_pfn(folio) && max_nr >= nr_pages)"

 just to account for cases where the user's end address falls within
the middle of a large folio?


BTW, another minor issue is here:

                if (++batch_count == SWAP_CLUSTER_MAX) {
                        batch_count = 0;
                        if (need_resched()) {
                                arch_leave_lazy_mmu_mode();
                                pte_unmap_unlock(start_pte, ptl);
                                cond_resched();
                                goto restart;
                        }
                }

We are increasing 1 for nr ptes, thus, we are holding PTL longer
than small folios case? we used to increase 1 for each PTE.
Does it matter?

>                 nr = folio_pte_batch(folio, addr, pte, ptent, max_nr,
>                                      fpb_flags, NULL);
>
> If we are not mapping the first page of the folio, then it can't be a full
> mapping, so no need to call folio_pte_batch(). Just split it.
>
> >
> >> +
> >> +                       if (nr < folio_nr_pages(folio)) {
> >> +                               int err;
> >> +
> >> +                               if (folio_estimated_sharers(folio) > 1)
> >> +                                       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 (!start_pte)
> >> +                                       break;
> >> +                               arch_enter_lazy_mmu_mode();
> >> +                               nr = 0;
> >> +                               continue;
> >> +                       }
> >>                 }
> >>
> >>                 /*
> >>                  * Do not interfere with other mappings of this folio and
> >> -                * non-LRU folio.
> >> +                * non-LRU folio. 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_test_lru(folio) || folio_mapcount(folio) != 1)
> >> +               if (!folio_test_lru(folio) ||
> >> +                   folio_mapcount(folio) != folio_nr_pages(folio))
> >>                         continue;
> >
> > This looks so perfect and is exactly what I wanted to achieve.
> >
> >>
> >>                 if (pageout_anon_only_filter && !folio_test_anon(folio))
> >>                         continue;
> >>
> >> -               VM_BUG_ON_FOLIO(folio_test_large(folio), folio);
> >> -
> >> -               if (!pageout && pte_young(ptent)) {
> >> -                       ptent = ptep_get_and_clear_full(mm, addr, pte,
> >> -                                                       tlb->fullmm);
> >> -                       ptent = pte_mkold(ptent);
> >> -                       set_pte_at(mm, addr, pte, ptent);
> >> -                       tlb_remove_tlb_entry(tlb, pte, addr);
> >> +               if (!pageout) {
> >> +                       for (; nr != 0; nr--, pte++, addr += PAGE_SIZE) {
> >> +                               if (ptep_test_and_clear_young(vma, addr, pte))
> >> +                                       tlb_remove_tlb_entry(tlb, pte, addr);
> >> +                       }
> >
> > This looks so smart. if it is not pageout, we have increased pte
> > and addr here; so nr is 0 and we don't need to increase again in
> > for (; addr < end; pte += nr, addr += nr * PAGE_SIZE)
> >
> > otherwise, nr won't be 0. so we will increase addr and
> > pte by nr.
>
> Indeed. I'm hoping that Lance is able to follow a similar pattern for
> madvise_free_pte_range().
>
>
> >
> >
> >>                 }
> >>
> >>                 /*
> >> --
> >> 2.25.1
> >>
> >
> > Overall, LGTM,
> >
> > Reviewed-by: Barry Song <v-songbaohua@oppo.com>
>
> Thanks!
>
>