Re: [RFC PATCH v2 2/4] madvise: Use notify-able API to clear and flush page table entries

[Yu Zhao]

On Tue, Jul 25, 2023 at 10:44 PM Yin Fengwei <fengwei.yin@xxxxxxxxx> wrote:
>
>
>
> On 7/26/23 11:26, Yu Zhao wrote:
> > On Tue, Jul 25, 2023 at 8:49 PM Yin Fengwei <fengwei.yin@xxxxxxxxx> wrote:
> >>
> >>
> >> On 7/25/23 13:55, Yu Zhao wrote:
> >>> On Fri, Jul 21, 2023 at 3:41 AM Yin Fengwei <fengwei.yin@xxxxxxxxx> 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@xxxxxxxxx>
> >>>> ---
> >>>>  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 = 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 = 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);
> >>>> -               }
> >>>> -
> >>>> +               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.
>
> From https://lore.kernel.org/lkml/20181029105515.GD14127@xxxxxxx/,
> the reason that arm64 didn't drop whole flush tlb in ptep_clear_flush_young()
> is to prevent the stale data in TLB. I suppose there is no correction issue
> there also.
>
> So why keep stale data in TLB in madvise_cold_or_pageout_pte_range() is fine?

Sorry, I'm not sure I understand your question here.

In this patch, you removed tlb_remove_tlb_entry(), so we don't need
struct mmu_gather *tlb any more.

If you are asking why I prefer ptep_clear_young_notify() (no flush),
which also doesn't need tlb_remove_tlb_entry(), then the answer is
that the TLB size doesn't scale like DRAM does: the gap has been
growing exponentially. So there is no way TLB can hold stale entries
long enough to cause a measurable effect on the A-bit. This isn't a
conjecture -- it's been proven conversely: we encountered bugs (almost
every year) caused by missing TLB flushes and resulting in data
corruption. They were never easy to reproduce, meaning stale entries
never stayed long in TLB.