From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 2EDB93EBF03 for ; Tue, 17 Mar 2026 15:05:31 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773759934; cv=none; b=X32HnN4eX0MA460fLwcNlql0A8/KrYEVFBxTZa0lzSz4vyEofZBPTotnZzmtjvfzHGcacrD1TIbpOyWtnK0TY8NtWSQ0QWDBXNF10pMTEzYTX2OyZPb+q3vr3T8Q6DhKFxYHMxGjj8guG6GFBLFG1KYg2WMr4nwybY3603pc/Xo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773759934; c=relaxed/simple; bh=3kelOZIoHcAp1rvQiB8ORqo403M6NhUepLDplZUhg+Q=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ARkkBJKEFsQu+DVx2/c0Td3u7nCvgzMsycZCyl4TDjA+eBOOO659VhdXRLPqrG8inf7yMBRYufMySoPeqLQfFMLTQ54v/tSBQOMxlLTf+oOmzVNtmRf7Ey9OMU6cOKg8jbmU33AwqtJmeCm/zuwvYMYSwr5YR5EZKini2RDIwDA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 4D4961477; Tue, 17 Mar 2026 08:05:25 -0700 (PDT) Received: from [10.1.37.182] (unknown [10.1.37.182]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 48ECB3F778; Tue, 17 Mar 2026 08:05:29 -0700 (PDT) Message-ID: Date: Tue, 17 Mar 2026 15:05:27 +0000 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v8 0/5] arm64: support FEAT_BBM level 2 and large block mapping when rodata=full Content-Language: en-GB To: Kevin Brodsky , Yang Shi , Jinjiang Tu , catalin.marinas@arm.com, will@kernel.org, akpm@linux-foundation.org, david@redhat.com, lorenzo.stoakes@oracle.com, ardb@kernel.org, dev.jain@arm.com, scott@os.amperecomputing.com, cl@gentwo.org Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20250917190323.3828347-1-yang@os.amperecomputing.com> <0b2a4ae5-fc51-4d77-b177-b2e9db74f11d@huawei.com> <0a740020-4780-4156-a9c5-f8b4ada9c8c0@os.amperecomputing.com> <4ad2ea40-b23b-4231-a0de-585b205865c5@arm.com> <9dded616-989b-4846-8596-1c45a6304d36@arm.com> <6c0ed052-5f3c-405a-b53f-4ea21a24479d@arm.com> <3d2e8e41-8b41-4d1d-9292-de90425708ec@arm.com> From: Ryan Roberts In-Reply-To: <3d2e8e41-8b41-4d1d-9292-de90425708ec@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 17/03/2026 12:43, Kevin Brodsky wrote: > On 17/03/2026 12:45, Ryan Roberts wrote: >> On 17/03/2026 09:29, Kevin Brodsky wrote: >>> On 17/03/2026 10:13, Ryan Roberts wrote: >>>>>>> Another option would be to initially map by pte then collapse to >>>>>>> block mappings >>>>>>> once we have determined that all cpus support BBML2_NOABORT. We >>>>>>> originally opted >>>>>>> not to do that because it's a tax on symetric systems. But we could >>>>>>> throw in the >>>>>>> towel if it's the least bad solution we can come up with for solving >>>>>>> this. I >>>>>>> think it might help some of Kevin's use cases too? >>>>>> May be an option too. When we discussed this there was no usecase for >>>>>> direct mapping collapse. But if we can have multiple usecases, it may >>>>>> be worth it. >>>> I could imagine that if user space creates and destroys lots of secretmem areas, >>>> then it will completely split the linear map to ptes and that will never recover >>>> currently. So I think in the long term, having the ability to collapse would be >>>> useful. I just don't particularly like forcing symetric systems to map by pte >>>> initially (which is slow) only to collapse later (which will cost even more >>>> time). But it does feel inherrently more robust. >>> Now that you spell it out, I'm realising this would actually make things >>> pretty complicated for protected page tables. In that series, page >>> tables for the linear map are allocated by a separate memblock-based >>> allocator [1], tracking the allocated ranges to set their pkey later. >>> There's a strong assumption that these page tables are never freed. >>> >>> If we initially PTE-mapped the linear map and then later collapsed it, >>> that assumption clearly wouldn't hold. >> Sorry I don't understand why the assumptions change? All I'm proposing is walkng >> the linear map to find compatible PTEs and collapsing them into the biggest >> possible blocks. The pages aren't being freed, they are just being mapped >> differently (which can be done live for BBML2_NOABORT). PTEs with different >> pkeys would be considered incompatible, so we would end up with a boundary in >> the leaf mappings at that point. > > I'm not sure I'm following, if all entries in a PTE page are compatible, > then surely we just convert the parent PMD entry to become a leaf and > then free the PTE page? And same idea one level above. Ahh - good point! That totally passed me by before. But I'm not sure it's the end of the world... We would end up with about 0.2% (4K/2M if I've done my maths correctly?) of the linear map sub-optimally mapped. Personally I don't think that would be the end of the world. > >> >>> It could be handled by poking >>> holes in the tracked ranges, but it gets ugly and increases fragmentation. >> You'd still want page tables to be allocated from contiguous physical (and >> virtual) memory so that the boundaries where pkeys change are minimized. > > Yes that's for sure, that's why I'm concerned with individual pages > being freed in a middle of a block. > >> I guess I've misunderstood something... > > I might have too :/ > > - Kevin