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

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Date:   Tue, 20 Jul 2021 13:53:39 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>
Cc:     Huang Ying <ying.huang@...el.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>, Zi Yan <ziy@...dia.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 Tue, Jul 20, 2021 at 7:25 AM Christian Borntraeger
<borntraeger@...ibm.com> wrote:
>
>
>
> On 20.07.21 08:55, Huang Ying wrote:
> > Before the commit c5b5a3dd2c1f ("mm: thp: refactor NUMA fault
> > handling"), the TLB flushing is done in do_huge_pmd_numa_page() itself
> > via flush_tlb_range().
> >
> > But after commit c5b5a3dd2c1f ("mm: thp: refactor NUMA fault
> > handling"), the TLB flushing is done in migrate_pages() as in the
> > following code path anyway.
> >
> > do_huge_pmd_numa_page
> >    migrate_misplaced_page
> >      migrate_pages
> >
> > So now, the TLB flushing code in do_huge_pmd_numa_page() becomes
> > unnecessary.  So the code is deleted in this patch to simplify the
> > code.  This is only code cleanup, there's no visible performance
> > difference.
> >
> > Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
> > Cc: Yang Shi <shy828301@...il.com>
> > Cc: Dan Carpenter <dan.carpenter@...cle.com>
> > Cc: Mel Gorman <mgorman@...e.de>
> > Cc: Christian Borntraeger <borntraeger@...ibm.com>
> > Cc: Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
> > Cc: Heiko Carstens <hca@...ux.ibm.com>
> > Cc: Hugh Dickins <hughd@...gle.com>
> > Cc: Andrea Arcangeli <aarcange@...hat.com>
> > Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > Cc: Michal Hocko <mhocko@...e.com>
> > Cc: Vasily Gorbik <gor@...ux.ibm.com>
> > Cc: Zi Yan <ziy@...dia.com>
> > ---
> >   mm/huge_memory.c | 26 --------------------------
> >   1 file changed, 26 deletions(-)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index afff3ac87067..9f21e44c9030 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -1440,32 +1440,6 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
> >               goto out;
> >       }
> >
> > -     /*
> > -      * Since we took the NUMA fault, we must have observed the !accessible
> > -      * bit. Make sure all other CPUs agree with that, to avoid them
> > -      * modifying the page we're about to migrate.
> > -      *
> > -      * Must be done under PTL such that we'll observe the relevant
> > -      * inc_tlb_flush_pending().
> > -      *
> > -      * We are not sure a pending tlb flush here is for a huge page
> > -      * mapping or not. Hence use the tlb range variant
> > -      */
> > -     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.

Zi Yan is the author for THP migration code, he may have some clue.

>
> >       pmd = pmd_modify(oldpmd, vma->vm_page_prot);
> >       page = vm_normal_page_pmd(vma, haddr, pmd);
> >       if (!page)
> >