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From: Sean Christopherson <seanjc@google.com>
To: Yang Shi <shy828301@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Huang Ying <ying.huang@intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux MM <linux-mm@kvack.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Dan Carpenter <dan.carpenter@oracle.com>,
	Mel Gorman <mgorman@suse.de>,
	Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
	Heiko Carstens <hca@linux.ibm.com>,
	Hugh Dickins <hughd@google.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Michal Hocko <mhocko@suse.com>, Vasily Gorbik <gor@linux.ibm.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	kvm list <kvm@vger.kernel.org>
Subject: Re: [PATCH] mm,do_huge_pmd_numa_page: remove unnecessary TLB flushing code
Date: Wed, 21 Jul 2021 15:41:05 +0000	[thread overview]
Message-ID: <YPhAEcHOCZ5yII/T@google.com> (raw)
In-Reply-To: <CAHbLzkqZZEic7+H0ky9u+aKO5o_cF0N5xQ=JO2tMpc8jg8RcnQ@mail.gmail.com>

On Tue, Jul 20, 2021, Yang Shi wrote:
> On Tue, Jul 20, 2021 at 2:04 PM Zi Yan <ziy@nvidia.com> wrote:
> >
> > On 20 Jul 2021, at 16:53, Yang Shi wrote:
> >
> > > On Tue, Jul 20, 2021 at 7:25 AM Christian Borntraeger
> > > <borntraeger@de.ibm.com> wrote:
> > >>> -     if (mm_tlb_flush_pending(vma->vm_mm)) {
> > >>> -             flush_tlb_range(vma, haddr, haddr + HPAGE_PMD_SIZE);
> > >>> -             /*
> > >>> -              * change_huge_pmd() released the pmd lock before
> > >>> -              * invalidating the secondary MMUs sharing the primary
> > >>> -              * MMU pagetables (with ->invalidate_range()). The
> > >>> -              * mmu_notifier_invalidate_range_end() (which
> > >>> -              * internally calls ->invalidate_range()) in
> > >>> -              * change_pmd_range() will run after us, so we can't
> > >>> -              * rely on it here and we need an explicit invalidate.
> > >>> -              */
> > >>> -             mmu_notifier_invalidate_range(vma->vm_mm, haddr,
> > >>> -                                           haddr + HPAGE_PMD_SIZE);
> > >>> -     }
> > >>> CC Paolo/KVM list so we also remove the mmu notifier here. Do we need those
> > >> now in migrate_pages? I am not an expert in that code, but I cant find
> > >> an equivalent mmu_notifier in migrate_misplaced_pages.
> > >> I might be totally wrong, just something that I noticed.
> > >
> > > Do you mean the missed mmu notifier invalidate for the THP migration
> > > case? Yes, I noticed that too. But I'm not sure whether it is intended
> > > or just missed.
> >
> > From my understand of mmu_notifier document, mmu_notifier_invalidate_range()
> > is needed only if the PTE is updated to point to a new page or the page pointed
> > by the PTE is freed. Page migration does not fall into either case.

The "new page" part of

  a page table entry is updated to point to a new page

is referring to a different physical page, i.e. a different pfn, not a different
struct page.  do_huge_pmd_numa_page() is moving a THP between nodes, thus it's
changing the backing pfn and needs to invalidate secondary MMUs at some point.

> > In addition, in migrate_pages(), more specifically try_to_migrate_one(),
> > there is a pair of mmu_notifier_invalidate_range_start() and
> > mmu_notifier_invalidate_range_end() around the PTE manipulation code, which should
> > be sufficient to notify secondary TLBs (including KVM) about the PTE change
> > for page migration. Correct me if I am wrong.
> 
> 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.

  reply	other threads:[~2021-07-21 15:41 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-20  6:55 Huang Ying
2021-07-20 13:36 ` Zi Yan
2021-07-20 14:25 ` Christian Borntraeger
2021-07-20 20:53   ` Yang Shi
2021-07-20 21:04     ` Zi Yan
2021-07-20 22:19       ` Yang Shi
2021-07-21 15:41         ` Sean Christopherson [this message]
2021-07-22  0:26           ` Huang, Ying
2021-07-22  7:36             ` Christian Borntraeger
2021-07-22 23:10               ` Huang, Ying
2021-07-23  0:03               ` Huang, Ying
2021-07-23 20:19                 ` Andrew Morton
2021-07-20 20:48 ` Yang Shi
2021-07-20 22:21   ` Yang Shi

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