Re: [PATCH] arm64: mm: drop tlb flush operation when clearing the access bit

["Yin, Fengwei" <fengwei.yin@intel.com>]

>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index 0bd18de9fd97..2979d796ba9d 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -905,21 +905,22 @@ static inline int ptep_test_and_clear_young(struct vm_area_struct *vma,
>>  static inline int ptep_clear_flush_young(struct vm_area_struct *vma,
>>                                          unsigned long address, pte_t *ptep)
>>  {
>> -       int young = ptep_test_and_clear_young(vma, address, ptep);
>> -
>> -       if (young) {
>> -               /*
>> -                * We can elide the trailing DSB here since the worst that can
>> -                * happen is that a CPU continues to use the young entry in its
>> -                * TLB and we mistakenly reclaim the associated page. The
>> -                * window for such an event is bounded by the next
>> -                * context-switch, which provides a DSB to complete the TLB
>> -                * invalidation.
>> -                */
>> -               flush_tlb_page_nosync(vma, address);
>> -       }
>> -
>> -       return young;
>> +       /*
>> +        * This comment is borrowed from x86, but applies equally to ARM64:
>> +        *
>> +        * Clearing the accessed bit without a TLB flush doesn't cause
>> +        * data corruption. [ It could cause incorrect page aging and
>> +        * the (mistaken) reclaim of hot pages, but the chance of that
>> +        * should be relatively low. ]
>> +        *
>> +        * So as a performance optimization don't flush the TLB when
>> +        * clearing the accessed bit, it will eventually be flushed by
>> +        * a context switch or a VM operation anyway. [ In the rare
>> +        * event of it not getting flushed for a long time the delay
>> +        * shouldn't really matter because there's no real memory
>> +        * pressure for swapout to react to. ]
>> +        */
>> +       return ptep_test_and_clear_young(vma, address, ptep);
>>  }
From https://lore.kernel.org/lkml/20181029105515.GD14127@arm.com/:

This is blindly copied from x86 and isn't true for us: we don't invalidate
the TLB on context switch. That means our window for keeping the stale
entries around is potentially much bigger and might not be a great idea.


My understanding is that arm64 doesn't do invalidate the TLB during
context switch. The flush_tlb_page_nosync() here + DSB during context
switch make sure the TLB is invalidated during context switch.
So we can't remove flush_tlb_page_nosync() here? Or something was changed
for arm64 (I have zero knowledge to TLB on arm64. So some obvious thing
may be missed)? Thanks.


Regards
Yin, Fengwei