linux-kernel - Re: [PATCH 6/9] mm/memory: split non-tlb flushing part from zap_page_range

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Message-ID: <6250fc68-2ce8-43a8-a064-e24877033ce1@lucifer.local>
Date: Tue, 11 Mar 2025 12:45:44 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: SeongJae Park <sj@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R. Howlett" <howlett@...il.com>,
        David Hildenbrand <david@...hat.com>,
        Shakeel Butt <shakeel.butt@...ux.dev>,
        Vlastimil Babka <vbabka@...e.cz>, kernel-team@...a.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 6/9] mm/memory: split non-tlb flushing part from
 zap_page_range_single()

On Mon, Mar 10, 2025 at 10:23:15AM -0700, SeongJae Park wrote:
> Some of zap_page_range_single() callers such as [process_]madvise() with
> MADV_DONEED[_LOCKED] cannot batch tlb flushes because
> zap_page_range_single() does tlb flushing for each invocation.  Split
> out the body of zap_page_range_single() except mmu_gather object
> initialization and gathered tlb entries flushing parts for such batched
> tlb flushing usage.
>
> Signed-off-by: SeongJae Park <sj@...nel.org>
> ---
>  mm/memory.c | 36 ++++++++++++++++++++++--------------
>  1 file changed, 22 insertions(+), 14 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 78c7ee62795e..88c478e2ed1a 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1995,38 +1995,46 @@ void unmap_vmas(struct mmu_gather *tlb, struct ma_state *mas,
>  	mmu_notifier_invalidate_range_end(&range);
>  }
>
> -/**
> - * zap_page_range_single - remove user pages in a given range
> - * @vma: vm_area_struct holding the applicable pages
> - * @address: starting address of pages to zap
> - * @size: number of bytes to zap
> - * @details: details of shared cache invalidation
> - *
> - * The range must fit into one VMA.
> - */
> -void zap_page_range_single(struct vm_area_struct *vma, unsigned long address,
> +static void unmap_vma_single(struct mmu_gather *tlb,
> +		struct vm_area_struct *vma, unsigned long address,
>  		unsigned long size, struct zap_details *details)
>  {
>  	const unsigned long end = address + size;
>  	struct mmu_notifier_range range;
> -	struct mmu_gather tlb;
>
>  	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma->vm_mm,
>  				address, end);
>  	hugetlb_zap_begin(vma, &range.start, &range.end);
> -	tlb_gather_mmu(&tlb, vma->vm_mm);
>  	update_hiwater_rss(vma->vm_mm);
>  	mmu_notifier_invalidate_range_start(&range);
>  	/*
>  	 * unmap 'address-end' not 'range.start-range.end' as range
>  	 * could have been expanded for hugetlb pmd sharing.
>  	 */
> -	unmap_single_vma(&tlb, vma, address, end, details, false);
> +	unmap_single_vma(tlb, vma, address, end, details, false);
>  	mmu_notifier_invalidate_range_end(&range);
> -	tlb_finish_mmu(&tlb);
>  	hugetlb_zap_end(vma, details);

Previously hugetlb_zap_end() would happen after tlb_finish_mmu(), now it happens
before?

This seems like a major problem with this change. If not you need to explain why
not in the commit message.

>  }
>
> +/**
> + * zap_page_range_single - remove user pages in a given range
> + * @vma: vm_area_struct holding the applicable pages
> + * @address: starting address of pages to zap
> + * @size: number of bytes to zap
> + * @details: details of shared cache invalidation
> + *
> + * The range must fit into one VMA.
> + */
> +void zap_page_range_single(struct vm_area_struct *vma, unsigned long address,
> +		unsigned long size, struct zap_details *details)
> +{
> +	struct mmu_gather tlb;
> +
> +	tlb_gather_mmu(&tlb, vma->vm_mm);
> +	unmap_vma_single(&tlb, vma, address, size, details);
> +	tlb_finish_mmu(&tlb);
> +}
> +
>  /**
>   * zap_vma_ptes - remove ptes mapping the vma
>   * @vma: vm_area_struct holding ptes to be zapped
> --
> 2.39.5