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

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Message-ID: <74603a7e-74f7-4d8f-9c2e-7caa42f2399f@arm.com>
Date: Thu, 4 Sep 2025 12:09:39 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Catalin Marinas <catalin.marinas@....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 03/09/2025 20:20, Catalin Marinas wrote:
> 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.

Yep, will do. Thanks for the review.