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 63F1B3C1419 for ; Tue, 17 Mar 2026 12:43:12 +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=1773751394; cv=none; b=lcFvp9yKuHj8dtNdLoeKOzqzm+fENdWwxA2SDZNfVKPkDhT9RlrMIIpH+XVIjelhtugUViJQR63HIvUUJYoft6YilP7Evgn6ISP4xzEMh3SnAePRM4FK44yLsjOY1wc5XtcaesTwLVx4MT1imgX8HgWkBPYoo+Gw/S+q2tGNkT8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773751394; c=relaxed/simple; bh=zEU0+pgG12cbowjYIkn2uSJdpkHwUReQ6wDQvJp2Tt8=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=VxGfYTz1DPrkFBRZjiWPdZAFcdvH4FCkqzJE1LsaXTcCv+Ldjsru7PV4bPfLYv8lzhGeqlEeTYWwoo1u1rSUG34rUSW+JGjEBgxS42cEDzd59LZHcLTg8r3kMfJ/ERofVDWOv3wSa7jSwnIB+CDWRbAX+6CpJ2RwD5AK4mrv8nE= 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 5B2B716F2; Tue, 17 Mar 2026 05:43:05 -0700 (PDT) Received: from [10.57.60.143] (unknown [10.57.60.143]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 5C9473F7BD; Tue, 17 Mar 2026 05:43:08 -0700 (PDT) Message-ID: <3d2e8e41-8b41-4d1d-9292-de90425708ec@arm.com> Date: Tue, 17 Mar 2026 13:43:05 +0100 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 To: Ryan Roberts , 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> From: Kevin Brodsky Content-Language: en-GB In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit 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. > >> 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