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

[Huang, Ying]

Peter Xu <peterx@xxxxxxxxxx> writes:

> On Wed, Aug 17, 2022 at 02:41:19AM -0700, Nadav Amit wrote:
>> 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/
>
> I need more reading on tlb code and also [3] which looks useful to me.
> It's definitely sad to make tlb flushing so complicated.  It'll be great if
> things can be sorted out someday.
>
> In this specific case, the only way to do safe tlb batching in my mind is:
>
> 	pte_offset_map_lock();
> 	arch_enter_lazy_mmu_mode();
>         // If any pending tlb, do it now
>         if (mm_tlb_flush_pending())
> 		flush_tlb_range(vma, start, end);
>         else
>                 flush_tlb_batched_pending();

I don't think we need the above 4 lines.  Because we will flush TLB
before we access the pages.  Can you find any issue if we don't use the
above 4 lines?

Best Regards,
Huang, Ying

>         loop {
>                 ...
>                 pte = ptep_get_and_clear();
>                 ...
>                 if (pte_present())
>                         unmapped++;
>                 ...
>         }
> 	if (unmapped)
> 		flush_tlb_range(walk->vma, start, end);
> 	arch_leave_lazy_mmu_mode();
> 	pte_unmap_unlock();
>
> I may miss something, but even if not it already doesn't look pretty.
>
> Thanks,