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

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Date:   Tue, 20 Jul 2021 16:25:37 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Huang Ying <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Yang Shi <shy828301@...il.com>,
        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 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.

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