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

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Date:   Tue, 20 Jul 2021 17:04:09 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Yang Shi <shy828301@...il.com>
Cc:     Christian Borntraeger <borntraeger@...ibm.com>,
        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>,
        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 20 Jul 2021, at 16:53, Yang Shi wrote:

> 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.

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.
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.

—
Best Regards,
Yan, Zi

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