Re: [PATCH] mm,do_huge_pmd_numa_page: remove unnecessary TLB flushing code

[Huang, Ying]

Sean Christopherson <seanjc@xxxxxxxxxx> writes:
>> 
>> Thanks, I think you are correct. By looking into commit 7066f0f933a1
>> ("mm: thp: fix mmu_notifier in migrate_misplaced_transhuge_page()"),
>> the tlb flush and mmu notifier invalidate were needed since the old
>> numa fault implementation didn't change PTE to migration entry so it
>> may cause data corruption due to the writes from GPU secondary MMU.
>> 
>> The refactor does use the generic migration code which converts PTE to
>> migration entry before copying data to the new page.
>
> That's my understanding as well, based on this blurb from commit 7066f0f933a1.
>
>     The standard PAGE_SIZEd migrate_misplaced_page is less accelerated and
>     uses the generic migrate_pages which transitions the pte from
>     numa/protnone to a migration entry in try_to_unmap_one() and flushes TLBs
>     and all mmu notifiers there before copying the page.
>
> That analysis/justification for removing the invalidate_range() call should be
> captured in the changelog.  Confirmation from Andrea would be a nice bonus.

When we flush CPU TLB for a page that may be shared with device/VM TLB,
we will call MMU notifiers for the page to flush the device/VM TLB.
Right?  So when we replaced CPU TLB flushing in do_huge_pmd_numa_page()
with that in try_to_migrate_one(), we will replace the MMU notifiers
calling too.  Do you agree?

Best Regards,
Huang, Ying