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

Lance Yang
2024-03-21T01:38:56Z

On Thu, Mar 21, 2024 at 1:38 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> On 20/03/2024 14:35, Lance Yang wrote:
> > 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))
>
>
> I'm still not really getting it; with my code, if nr < the folio size, we will
> try to split and if we estimate that the folio is not exclusive we will avoid
> locking the folio, etc. If nr == folio size, we will proceed to the precise
> exclusivity check (which is cheap once we know the folio is fully mapped by this
> process).
>
> With your change, we will always do the estimated exclusive check then proceed
> to the precise check; seems like duplication to me?

Agreed. The estimated exclusive check is indeed redundant with my change.

>
> >
> >>
> >> 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().
>
> Err, I assumed tlb_remove_tlb_entry() meant "invalidate the TLB entry for this
> address please" - albeit its deferred and batched. I'll look into this.
>
> >
> > In my new patch[1], I use refresh_full_ptes() and
> > tlb_remove_tlb_entries() to batch-update the
> > access and dirty bits.
>
> I want to avoid the per-pte clear-modify-set approach, because this doesn't
> perform well on arm64 when using contpte mappings; it will cause the contpe
> mapping to be unfolded by the first clear that touches the contpte block, then
> refolded by the last set to touch the block. That's expensive.
> ptep_test_and_clear_young() doesn't suffer that problem.

Thanks for explaining. I got it.

I think that other architectures will benefit from the per-pte clear-modify-set
approach. IMO, refresh_full_ptes() can be overridden by arm64.

Thanks,
Lance
>
> >
> > [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!
> >>>>
> >>>>
> >>
>