linux-kernel - Re: [PATCH v7 4/6] arm64: mm: Optimize split_kernel_leaf

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Message-ID: <e004718a-52af-401d-8f22-c425948dd3ee@os.amperecomputing.com>
Date: Fri, 29 Aug 2025 15:11:11 -0700
From: Yang Shi <yang@...amperecomputing.com>
To: Ryan Roberts <ryan.roberts@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...hat.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Ard Biesheuvel <ardb@...nel.org>, Dev Jain <dev.jain@....com>,
 scott@...amperecomputing.com, cl@...two.org
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [PATCH v7 4/6] arm64: mm: Optimize split_kernel_leaf_mapping()



On 8/29/25 4:52 AM, Ryan Roberts wrote:
> The common case for split_kernel_leaf_mapping() is for a single page.
> Let's optimize this by only calling split_kernel_leaf_mapping_locked()
> once.
>
> Since the start and end address are PAGE_SIZE apart, they must be
> contained within the same contpte block. Further, if start is at the
> beginning of the block or end is at the end of the block, then the other
> address must be in the _middle_ of the block. So if we split on this
> middle-of-the-contpte-block address, it is guaranteed that the
> containing contpte block is split to ptes and both start and end are
> therefore mapped by pte.
>
> This avoids the second call to split_kernel_leaf_mapping_locked()
> meaning we only have to walk the pgtable once.
>
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> ---
>   arch/arm64/mm/mmu.c | 18 +++++++++++++++---
>   1 file changed, 15 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 114b88216b0c..8b5b19e1154b 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -740,9 +740,21 @@ int split_kernel_leaf_mapping(unsigned long start, unsigned long end)
>   	mutex_lock(&pgtable_split_lock);
>   	arch_enter_lazy_mmu_mode();
>   
> -	ret = split_kernel_leaf_mapping_locked(start);
> -	if (!ret)
> -		ret = split_kernel_leaf_mapping_locked(end);
> +	/*
> +	 * Optimize for the common case of splitting out a single page from a
> +	 * larger mapping. Here we can just split on the "least aligned" of
> +	 * start and end and this will guarantee that there must also be a split
> +	 * on the more aligned address since the both addresses must be in the
> +	 * same contpte block and it must have been split to ptes.
> +	 */
> +	if (end - start == PAGE_SIZE) {
> +		start = __ffs(start) < __ffs(end) ? start : end;
> +		ret = split_kernel_leaf_mapping_locked(start);

This makes sense to me. I suggested the same thing in the discussion 
with Dev for v5. I'd like to have this patch squashed into patch #3.

Thanks,
Yang

> +	} else {
> +		ret = split_kernel_leaf_mapping_locked(start);
> +		if (!ret)
> +			ret = split_kernel_leaf_mapping_locked(end);
> +	}
>   
>   	arch_leave_lazy_mmu_mode();
>   	mutex_unlock(&pgtable_split_lock);