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

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Date:   Tue, 20 Jul 2021 15:21:06 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Huang Ying <ying.huang@...el.com>
Cc:     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>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        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>
Subject: Re: [PATCH] mm,do_huge_pmd_numa_page: remove unnecessary TLB flushing code

On Tue, Jul 20, 2021 at 1:48 PM Yang Shi <shy828301@...il.com> wrote:
>
> On Mon, Jul 19, 2021 at 11:56 PM Huang Ying <ying.huang@...el.com> 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.
>
> Yes, there is tlb flush in try_to_migrate(), but it seems mmu notifier
> invalidate is missed for the THP migration case. I'm not quite sure
> why it is not needed, maybe just missed?
>
> So, you may need the below change too:
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 2d29a57d29e8..e1c8b654563d 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1749,6 +1749,8 @@ static bool try_to_migrate_one(struct page
> *page, struct vm_area_struct *vma,
>                                        !PageTransCompound(page), page);
>
>                         set_pmd_migration_entry(&pvmw, page);
> +                       mmu_notifier_invalidate_range(mm, range.start,
> +                                                     range.end);
>                         continue;
>                 }
>  #endif

Per the discussion with Zi Yan the mmu notifier invalidate should be
not needed. The patch looks good to me. Reviewed-by: Yang Shi
<shy828301@...il.com>

>
> >
> > 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);
> > -       }
> > -
> >         pmd = pmd_modify(oldpmd, vma->vm_page_prot);
> >         page = vm_normal_page_pmd(vma, haddr, pmd);
> >         if (!page)
> > --
> > 2.30.2
> >