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

From: Huang, Ying
Date: Thu Jul 22 2021 - 00:02:09 EST

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