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

[Yin Fengwei]

On 7/26/23 13:40, Yu Zhao wrote:
> 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.
Oh. Sorry for the confusion. I will explain my understanding and
question in detail.

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

> 
> 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.

when I read https://lore.kernel.org/lkml/20181029105515.GD14127@xxxxxxx/,

my understanding is that arm64 still keep something in ptep_clear_flush_young.
The reason is finishing TLB flush by next context-switch to make sure no
stale entries in TLB cross next context switch.

Should we still keep same behavior (no stable entries in TLB cross next
context switch) for madvise_cold_or_pageout_pte_range()?

So two versions work (I assume we should keep same behavior) for me:
  1. using xxx_flush_xxx() version. but with possible regression on arm64.
  2. using none flush version. But do batched TLB flush.

Hope this could make things clear. Thanks.

Regards
Yin, Fengwei