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

[Nadav Amit <nadav.amit@gmail.com>]

On Aug 15, 2022, at 1:52 PM, Nadav Amit <nadav.amit@gmail.com> wrote:

> On Aug 15, 2022, at 12:18 PM, Peter Xu <peterx@redhat.com> wrote:
> 
>> On Fri, Aug 12, 2022 at 10:32:48AM +0800, Huang, Ying wrote:
>>> Peter Xu <peterx@redhat.com> 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@viggo.jf.intel.com/

I keep not remembering this erratum correctly. IIRC, the erratum says that
the access/dirty might be set, but it does not mean that a write is possible
after the PTE is cleared (i.e., the dirty/access might be set on the
non-present PTE, but the access itself would fail). So it is not an issue in
this case - losing A/D would not impact correctness since the access should
fail.

Dave Hansen hates when I get confused with this one, but I cc him if he
wants to confirm.

[ Having said all of that, in general the lack of regard to
  mm->tlb_flush_pending is always concerning in such functions. ]