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From:   Yu Zhao <yuzhao@google.com>
Date:   Tue, 25 Jul 2023 21:26:54 -0600
Message-ID: <CAOUHufYJE40wcT4HTYFJ_7X5=my3OPbMyMBt+QNZdByuL6j58Q@mail.gmail.com>
Subject: Re: [RFC PATCH v2 2/4] madvise: Use notify-able API to clear and
 flush page table entries
To:     Yin Fengwei <fengwei.yin@intel.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        akpm@linux-foundation.org, minchan@kernel.org, willy@infradead.org,
        david@redhat.com, ryan.roberts@arm.com, shy828301@gmail.com
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On Tue, Jul 25, 2023 at 8:49=E2=80=AFPM Yin Fengwei <fengwei.yin@intel.com>=
 wrote:
>
>
> On 7/25/23 13:55, Yu Zhao wrote:
> > On Fri, Jul 21, 2023 at 3:41=E2=80=AFAM Yin Fengwei <fengwei.yin@intel.=
com> wrote:
> >>
> >> Currently, in function madvise_cold_or_pageout_pte_range(), the
> >> young bit of pte/pmd is cleared notify subscripter.
> >>
> >> Using notify-able API to make sure the subscripter is signaled about
> >> the young bit clearing.
> >>
> >> Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>
> >> ---
> >>  mm/madvise.c | 18 ++----------------
> >>  1 file changed, 2 insertions(+), 16 deletions(-)
> >>
> >> diff --git a/mm/madvise.c b/mm/madvise.c
> >> index f12933ebcc24..b236e201a738 100644
> >> --- a/mm/madvise.c
> >> +++ b/mm/madvise.c
> >> @@ -403,14 +403,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_=
t *pmd,
> >>                         return 0;
> >>                 }
> >>
> >> -               if (pmd_young(orig_pmd)) {
> >> -                       pmdp_invalidate(vma, addr, pmd);
> >> -                       orig_pmd =3D pmd_mkold(orig_pmd);
> >> -
> >> -                       set_pmd_at(mm, addr, pmd, orig_pmd);
> >> -                       tlb_remove_pmd_tlb_entry(tlb, pmd, addr);
> >> -               }
> >> -
> >> +               pmdp_clear_flush_young_notify(vma, addr, pmd);
> >>                 folio_clear_referenced(folio);
> >>                 folio_test_clear_young(folio);
> >>                 if (folio_test_active(folio))
> >> @@ -496,14 +489,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_=
t *pmd,
> >>
> >>                 VM_BUG_ON_FOLIO(folio_test_large(folio), folio);
> >>
> >> -               if (pte_young(ptent)) {
> >> -                       ptent =3D ptep_get_and_clear_full(mm, addr, pt=
e,
> >> -                                                       tlb->fullmm);
> >> -                       ptent =3D pte_mkold(ptent);
> >> -                       set_pte_at(mm, addr, pte, ptent);
> >> -                       tlb_remove_tlb_entry(tlb, pte, addr);
> >> -               }
> >> -
> >> +               ptep_clear_flush_young_notify(vma, addr, pte);
> >
> > These two places are tricky.
> >
> > I agree there is a problem here, i.e., we are not consulting the mmu
> > notifier. In fact, we do pageout on VMs on ChromeOS, and it's been a
> > known problem to me for a while (not a high priority one).
> >
> > tlb_remove_tlb_entry() is batched flush, ptep_clear_flush_young() is
> > not. But, on x86, we might see a performance improvement since
> > ptep_clear_flush_young() doesn't flush TLB at all. On ARM, there might
> > be regressions though.
> >
> > I'd go with ptep_clear_young_notify(), but IIRC, Minchan mentioned he
> > prefers flush. So I'll let him chime in.
> I am OK with either way even no flush way here is more efficient for
> arm64. Let's wait for Minchan's comment.

Yes, and I don't think there would be any "negative" consequences
without tlb flushes when clearing the A-bit.

> > If we do end up with ptep_clear_young_notify(), please remove
> > mmu_gather -- it should have been done in this patch.
>
> I suppose "remove mmu_gather" means to trigger flush tlb operation in
> batched way to make sure no stale data in TLB for long time on arm64
> platform.

In madvise_cold_or_pageout_pte_range(), we only need struct
mmu_gather *tlb because of tlb_remove_pmd_tlb_entry(), i.e., flushing
tlb after clearing the A-bit. There is no correction, e.g., potential
data corruption, involved there.