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

[Nadav Amit]

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.

At least on x86, the hardware is not supposed to do so. The only case I
remember (and sometimes misremembers) is with KNL erratum, which perhaps
needs to be considered:

https://lore.kernel.org/all/20160708001911.9A3FD2B6@xxxxxxxxxxxxxxxxxx/