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

[Lance Yang <ioworker0@gmail.com>]

On Wed, Mar 20, 2024 at 9:49 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> Hi Lance, Barry,
>
> Sorry - I totally missed this when you originally sent it!

No worries at all :)

>
>
> On 13/03/2024 14:02, Lance Yang wrote:
> > On Wed, Mar 13, 2024 at 5:03 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)
> >>                 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 (folio_test_large(folio)) {
> > [...]
> >                        nr = folio_pte_batch(folio, addr, pte, ptent, max_nr,
> >                                             fpb_flags, NULL);
> > +                       if (folio_estimated_sharers(folio) > 1)
> > +                               continue;
> >
> > Could we use folio_estimated_sharers as an early exit point here?
>
> I'm not sure what this is saving where you have it? Did you mean to put it
> before folio_pte_batch()? Currently it is just saving a single conditional.

Apologies for the confusion. I made a diff to provide clarity.

diff --git a/mm/madvise.c b/mm/madvise.c
index 56c7ba7bd558..c3458fdea82a 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -462,12 +462,11 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,

                        nr = folio_pte_batch(folio, addr, pte, ptent, max_nr,
                                             fpb_flags, NULL);
-
// Could we use folio_estimated_sharers as an early exit point here?
+                       if (folio_estimated_sharers(folio) > 1)
+                               continue;
                        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))

>
> But now that I think about it a bit more, I remember why I was originally
> unconditionally calling folio_pte_batch(). Given its a large folio, if the split
> fails, we can move the cursor to the pte where the next folio begins so we don't
> have to iterate through one pte at a time which would cause us to keep calling
> folio_estimated_sharers(), folio_test_anon(), etc on the same folio until we get
> to the next boundary.
>
> Of course the common case at this point will be for the split to succeed, but
> then we are going to iterate over ever single PTE anyway - one way or another
> they are all fetched into cache. So I feel like its neater not to add the
> conditionals for calling folio_pte_batch(), and just leave this as I have it here.
>
> >
> >                        if (nr < folio_nr_pages(folio)) {
> >                                int err;
> >
> > -                               if (folio_estimated_sharers(folio) > 1)
> > -                                       continue;
> > [...]
> >
> >>
> >>>
> >>>> +
> >>>> +                       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);
> >
> > IIRC, some of the architecture(ex, PPC) don't update TLB with set_pte_at and
> > tlb_remove_tlb_entry. So, didn't we consider remapping the PTE with old after
> > pte clearing?
>
> Sorry Lance, I don't understand this question, can you rephrase? Are you saying
> there is a good reason to do the original clear-mkold-set for some arches?

IIRC, some of the architecture(ex, PPC)  don't update TLB with
ptep_test_and_clear_young()
and tlb_remove_tlb_entry().

In my new patch[1], I use refresh_full_ptes() and
tlb_remove_tlb_entries() to batch-update the
access and dirty bits.

[1] https://lore.kernel.org/linux-mm/20240316102952.39233-1-ioworker0@gmail.com

Thanks,
Lance

>
> >
> > Thanks,
> > Lance
> >
> >
> >
> >>>> +                       }
> >>>
> >>> 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!
> >>
> >>
>