[PATCH v2 1/2] mm/migrate_device.c: Copy pte dirty bit to page

[Huang, Ying]

Alistair Popple <apopple at nvidia.com> writes:

> Peter Xu <peterx at redhat.com> writes:
>
>> On Wed, Aug 17, 2022 at 11:49:03AM +1000, Alistair Popple wrote:
>>>
>>> Peter Xu <peterx at redhat.com> writes:
>>>
>>> > On Tue, Aug 16, 2022 at 04:10:29PM +0800, huang ying wrote:
>>> >> > @@ -193,11 +194,10 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>>> >> >                         bool anon_exclusive;
>>> >> >                         pte_t swp_pte;
>>> >> >
>>> >> > +                       flush_cache_page(vma, addr, pte_pfn(*ptep));
>>> >> > +                       pte = ptep_clear_flush(vma, addr, ptep);
>>> >>
>>> >> Although I think it's possible to batch the TLB flushing just before
>>> >> unlocking PTL.  The current code looks correct.
>>> >
>>> > If we're with unconditionally ptep_clear_flush(), does it mean we should
>>> > probably drop the "unmapped" and the last flush_tlb_range() already since
>>> > they'll be redundant?
>>>
>>> This patch does that, unless I missed something?
>>
>> Yes it does.  Somehow I didn't read into the real v2 patch, sorry!
>>
>>>
>>> > If that'll need to be dropped, it looks indeed better to still keep the
>>> > batch to me but just move it earlier (before unlock iiuc then it'll be
>>> > safe), then we can keep using ptep_get_and_clear() afaiu but keep "pte"
>>> > updated.
>>>
>>> I think we would also need to check should_defer_flush(). Looking at
>>> try_to_unmap_one() there is this comment:
>>>
>>> 			if (should_defer_flush(mm, flags) && !anon_exclusive) {
>>> 				/*
>>> 				 * We clear the PTE but do not flush so potentially
>>> 				 * a remote CPU could still be writing to the folio.
>>> 				 * If the entry was previously clean then the
>>> 				 * architecture must guarantee that a clear->dirty
>>> 				 * transition on a cached TLB entry is written through
>>> 				 * and traps if the PTE is unmapped.
>>> 				 */
>>>
>>> And as I understand it we'd need the same guarantee here. Given
>>> try_to_migrate_one() doesn't do batched TLB flushes either I'd rather
>>> keep the code as consistent as possible between
>>> migrate_vma_collect_pmd() and try_to_migrate_one(). I could look at
>>> introducing TLB flushing for both in some future patch series.
>>
>> should_defer_flush() is TTU-specific code?
>
> I'm not sure, but I think we need the same guarantee here as mentioned
> in the comment otherwise we wouldn't see a subsequent CPU write that
> could dirty the PTE after we have cleared it but before the TLB flush.
>
> My assumption was should_defer_flush() would ensure we have that
> guarantee from the architecture, but maybe there are alternate/better
> ways of enforcing that?
>> IIUC the caller sets TTU_BATCH_FLUSH showing that tlb can be omitted since
>> the caller will be responsible for doing it.  In migrate_vma_collect_pmd()
>> iiuc we don't need that hint because it'll be flushed within the same
>> function but just only after the loop of modifying the ptes.  Also it'll be
>> with the pgtable lock held.
>
> Right, but the pgtable lock doesn't protect against HW PTE changes such
> as setting the dirty bit so we need to ensure the HW does the right
> thing here and I don't know if all HW does.

This sounds sensible.  But I take a look at zap_pte_range(), and find
that it appears that the implementation requires the PTE dirty bit to be
write-through.  Do I miss something?

Hi, Nadav, Can you help?

Best Regards,
Huang, Ying

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