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

[Christian Borntraeger]

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@xxxxxxxxx>
> Cc: Yang Shi <shy828301@xxxxxxxxx>
> Cc: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> Cc: Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx>
> Cc: Heiko Carstens <hca@xxxxxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Vasily Gorbik <gor@xxxxxxxxxxxxx>
> Cc: Zi Yan <ziy@xxxxxxxxxx>
> ---
>   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)