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

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Message-ID: <572f1ddd-9ac6-fb09-9a24-1c667dbd1d03@de.ibm.com>
Date:   Thu, 22 Jul 2021 09:36:07 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     "Huang, Ying" <ying.huang@...el.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Yang Shi <shy828301@...il.com>, Zi Yan <ziy@...dia.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



On 22.07.21 02:26, Huang, Ying wrote:
> 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?

Can someone write an updated commit messages that contains this information?