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

[Yin Fengwei]

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.

> 
> 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.


Regards
Yin, Fengwei