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

[Yin Fengwei]

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?


Regards
Yin, Fengwei