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

Barry Song
2024-03-13T10:37:32Z

On Wed, Mar 13, 2024 at 10:36 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> On 13/03/2024 09:16, Barry Song wrote:
> > 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?
>
> yes, even better. I'll add this for the next version.
>
> >
> >
> > 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?
>
> I thought about that, but the vast majority of the work is per-folio, not
> per-pte. So I concluded it would be best to continue to increment per-folio.

Okay. The original patch commit b2f557a21bc8 ("mm/madvise: add
cond_resched() in madvise_cold_or_pageout_pte_range()")
primarily addressed the real-time wake-up latency issue. MADV_PAGEOUT
and MADV_COLD are much less critical compared
to other scenarios where operations like do_anon_page or do_swap_page
necessarily need PTL to progress. Therefore, adopting
an approach that relatively aggressively releases the PTL seems to
neither harm MADV_PAGEOUT/COLD nor disadvantage
others.

We are slightly increasing the duration of holding the PTL due to the
iteration of folio_pte_batch() potentially taking longer than
the case of small folios, which do not require it. However, compared
to operations like folio_isolate_lru() and folio_deactivate(),
this increase seems negligible. Recently, we have actually removed
ptep_test_and_clear_young() for MADV_PAGEOUT,
which should also benefit real-time scenarios. Nonetheless, there is a
small risk with large folios, such as 1 MiB mTHP, where
we may need to loop 256 times in folio_pte_batch().

I would vote for increasing 'nr' or maybe max(log2(nr), 1) rather than
1 for two reasons:

1. We are not making MADV_PAGEOUT/COLD worse; in fact, we are
improving them by reducing the time taken to put the same
number of pages into the reclaim list.

2. MADV_PAGEOUT/COLD scenarios are not urgent compared to others that
genuinely require the PTL to progress. Moreover,
the majority of time spent on PAGEOUT is actually reclaim_pages().

> >
> >> 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!
> >>

Thanks
Barry