Re: [PATCH v3 5/7] mm: Remember young/dirty bit for page migrations

[Huang, Ying]

Nadav Amit <nadav.amit@xxxxxxxxx> writes:

> On Aug 15, 2022, at 12:18 PM, Peter Xu <peterx@xxxxxxxxxx> wrote:
>
>> On Fri, Aug 12, 2022 at 10:32:48AM +0800, Huang, Ying wrote:
>>> Peter Xu <peterx@xxxxxxxxxx> writes:
>>> 
>>>> On Tue, Aug 09, 2022 at 06:00:58PM -0400, Peter Xu wrote:
>>>>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
>>>>> index 27fb37d65476..699f821b8443 100644
>>>>> --- a/mm/migrate_device.c
>>>>> +++ b/mm/migrate_device.c
>>>>> @@ -221,6 +221,10 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>>>>> 			else
>>>>> 				entry = make_readable_migration_entry(
>>>>> 							page_to_pfn(page));
>>>>> +			if (pte_young(pte))
>>>>> +				entry = make_migration_entry_young(entry);
>>>>> +			if (pte_dirty(pte))
>>>>> +				entry = make_migration_entry_dirty(entry);
>>>>> 			swp_pte = swp_entry_to_pte(entry);
>>>>> 			if (pte_present(pte)) {
>>>>> 				if (pte_soft_dirty(pte))
>>>> 
>>>> This change needs to be wrapped with pte_present() at least..
>>>> 
>>>> I also just noticed that this change probably won't help anyway because:
>>>> 
>>>>  (1) When ram->device, the pte will finally be replaced with a device
>>>>      private entry, and device private entry does not yet support A/D, it
>>>>      means A/D will be dropped again,
>>>> 
>>>>  (2) When device->ram, we are missing information on either A/D bits, or
>>>>      even if device private entries start to suport A/D, it's still not
>>>>      clear whether we should take device read/write into considerations
>>>>      too on the page A/D bits to be accurate.
>>>> 
>>>> I think I'll probably keep the code there for completeness, but I think it
>>>> won't really help much until more things are done.
>>> 
>>> It appears that there are more issues.  Between "pte = *ptep" and pte
>>> clear, CPU may set A/D bit in PTE, so we may need to update pte when
>>> clearing PTE.
>> 
>> Agreed, I didn't see it a huge problem with current code, but it should be
>> better in that way.
>> 
>>> And I don't find the TLB is flushed in some cases after PTE is cleared.
>> 
>> I think it's okay to not flush tlb if pte not present.  But maybe you're
>> talking about something else?
>
> I think Huang refers to situation in which the PTE is cleared, still not
> flushed, and then A/D is being set by the hardware.

No.  The situation in my mind is PTE with A/D set is cleared, not
flushed.  Then a parallel mprotect or munmap may cause race conditions.
As Alistair pointed out in another thread [1], there is TLB flushing
after PTL unlocked.  But I think we need to flush TLB before unlock.
This has been fixed in Alistair's latest version [2].

[1] https://lore.kernel.org/lkml/87r11gvrx6.fsf@nvdebian.thelocal/
[2] https://lore.kernel.org/lkml/6e77914685ede036c419fa65b6adc27f25a6c3e9.1660635033.git-series.apopple@xxxxxxxxxx/

Best Regards,
Huang, Ying