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

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Message-ID: <aLiU9xnZl-7LBqkX@arm.com>
Date: Wed, 3 Sep 2025 20:20:23 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Will Deacon <will@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Hildenbrand <david@...hat.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Yang Shi <yang@...amperecomputing.com>,
	Ard Biesheuvel <ardb@...nel.org>, Dev Jain <dev.jain@....com>,
	scott@...amperecomputing.com, cl@...two.org,
	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 Fri, Aug 29, 2025 at 12:52:45PM +0100, 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>

Reviewed-by: Catalin Marinas <catalin.marinas@....com>

And I agree with Yang, you can just fold this into the previous patch.