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

[Nadav Amit]

On Aug 17, 2022, at 12:17 AM, Huang, Ying <ying.huang@xxxxxxxxx> wrote:

> Alistair Popple <apopple@xxxxxxxxxx> writes:
> 
>> Peter Xu <peterx@xxxxxxxxxx> writes:
>> 
>>> On Wed, Aug 17, 2022 at 11:49:03AM +1000, Alistair Popple wrote:
>>>> Peter Xu <peterx@xxxxxxxxxx> 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?

Sorry for joining the discussion late. I read most ofthis thread and I hope
I understand what you ask me. So at the risk of rehashing or repeating what
you already know - here are my 2 cents. Feel free to ask me again if I did
not understand your questions:

1. ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH is currently x86 specific. There is a
recent patch that want to use it for arm64 as well [1]. The assumption that
Alistair cited from the code (regarding should_defer_flush()) might not be
applicable to certain architectures (although most likely it is). I tried
to encapsulate the logic on whether an unflushed RO entry can become dirty
in an arch specific function [2].

2. Having said all of that, using the logic of “flush if there are pending
TLB flushes for this mm” as done by UNMAP_TLB_FLUSH makes sense IMHO
(although I would have considered doing it in finer granularity of
VMA/page-table as I proposed before and got somewhat lukewarm response [3]).

3. There is no question that flushing after dropping the ptl is wrong. But
reading the thread, I think that you only focus on whether a dirty
indication might get lost. The problem, I think, is bigger, as it might also
cause correction problems after concurrently removing mappings. What happens
if we get for a clean PTE something like:

  CPU0					CPU1
  ----					----

  migrate_vma_collect_pmd()
  [ defer flush, release ptl ]
					madvise(MADV_DONTNEED)
					-> zap_pte_range()
					[ PTE not present; mmu_gather
					  not updated ]
					
					[ no flush; stale PTE in TLB ]

					[ page is still accessible ]

[ might apply to munmap(); I usually regard MADV_DONTNEED since it does
  not call mmap_write_lock() ]

4. Having multiple TLB flushing infrastructures makes all of these
discussions very complicated and unmaintainable. I need to convince myself
in every occasion (including this one) whether calls to
flush_tlb_batched_pending() and tlb_flush_pending() are needed or not.

What I would like to have [3] is a single infrastructure that gets a
“ticket” (generation when the batching started), the old PTE and the new PTE
and checks whether a TLB flush is needed based on the arch behavior and the
current TLB generation. If needed, it would update the “ticket” to the new
generation. Andy wanted a ring for pending TLB flushes, but I think it is an
overkill with more overhead and complexity than needed.

But the current situation in which every TLB flush is a basis for long
discussions and prone to bugs is impossible.

I hope it helps. Let me know if you want me to revive the patch-set or other
feedback.

[1] https://lore.kernel.org/all/20220711034615.482895-5-21cnbao@xxxxxxxxx/
[2] https://lore.kernel.org/all/20220718120212.3180-13-namit@xxxxxxxxxx/
[3] https://lore.kernel.org/all/20210131001132.3368247-16-namit@xxxxxxxxxx/