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

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Date:   Thu, 22 Jul 2021 08:26:34 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Yang Shi <shy828301@...il.com>, Zi Yan <ziy@...dia.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Mel Gorman <mgorman@...e.de>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Hugh Dickins <hughd@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        kvm list <kvm@...r.kernel.org>
Subject: Re: [PATCH] mm,do_huge_pmd_numa_page: remove unnecessary TLB
 flushing code

Sean Christopherson <seanjc@...gle.com> 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