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 05BC133CE81; Wed, 26 Nov 2025 16:07:28 +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=1764173252; cv=none; b=e8b42pqVtHk9VJOCfLSErEJ7UYJ66LC9u4SY5sXBN/Ye7WkL4Jl6f4xnmqehsawiW6VNrNHr2UpYH22idYFBUtZCdfTVS0E47ETbpWAeMVNIvSvbgX3wdK1Q1oIrIKRnrmqbNNOi1E+J2a+RHBj6jHEdIt1R5Zo8HEtAb5xVvdw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764173252; c=relaxed/simple; bh=D5u21sgtki/irD2XESXFalufjgS/F69R9LlqZDg31ZE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=fVdYa72jzM6qQox6ptNeP34OPBreYEsZ4geF3hlLd8i+/2BBYuTclC1Rl3DmqQ2lrIYs/lt0djorva2FCPnxpYkuPgrCeDJdRJIpz8rkT6dg3CHyybRwXpo6pRtg+FcLG4gwTiA2Z33bLlVXnsgauvwuBzjDazS3NeFGZqzywQo= 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 4B9C41692; Wed, 26 Nov 2025 08:07:20 -0800 (PST) Received: from [10.1.33.153] (unknown [10.1.33.153]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 051ED3F73B; Wed, 26 Nov 2025 08:07:24 -0800 (PST) Message-ID: <4505a93b-2bac-4ce1-8971-4c31f1ce1362@arm.com> Date: Wed, 26 Nov 2025 16:07:23 +0000 Precedence: bulk X-Mailing-List: devicetree@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 06/22] mm: Always use page table accessor functions Content-Language: en-GB To: "David Hildenbrand (Red Hat)" , Lorenzo Stoakes Cc: Wei Yang , Samuel Holland , Palmer Dabbelt , Paul Walmsley , linux-riscv@lists.infradead.org, Andrew Morton , linux-mm@kvack.org, devicetree@vger.kernel.org, Suren Baghdasaryan , linux-kernel@vger.kernel.org, Mike Rapoport , Michal Hocko , Conor Dooley , Krzysztof Kozlowski , Alexandre Ghiti , Emil Renner Berthing , Rob Herring , Vlastimil Babka , "Liam R . Howlett" , Julia Lawall , Nicolas Palix , Anshuman Khandual References: <6bdf2b89-7768-4b90-b5e7-ff174196ea7b@lucifer.local> <71123d7a-641b-41df-b959-88e6c2a3a441@kernel.org> <20251126134726.yrya5xxayfcde3kl@master> <6b966403-91e0-4f06-86a9-a4f7780b9557@kernel.org> <1ca9f99f-6266-47ca-8c94-1a9b9aaa717f@kernel.org> <37973e21-e8f4-4603-b93d-4e0b1b2499fa@lucifer.local> From: Ryan Roberts In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 26/11/2025 15:12, David Hildenbrand (Red Hat) wrote: > On 11/26/25 16:08, Lorenzo Stoakes wrote: >> On Wed, Nov 26, 2025 at 03:56:13PM +0100, David Hildenbrand (Red Hat) wrote: >>> On 11/26/25 15:52, Lorenzo Stoakes wrote: >>>> >>>> Would the pmdp_get() never get invoked then? Or otherwise wouldn't that end up >>>> requiring a READ_ONCE() further up the stack? >>> >>> See my other reply, I think the pmdp_get() is required because all pud_* >>> functions are just simple stubs. >> >> OK, thought you were saying we should push further down the stack? Or up >> depending on how you view these things :P as in READ_ONCE at leaf? > > I think at leaf because I think the previous ones should essentially be only > used by stubs. > > But I haven't fully digested how this is all working. Or supposed to work. > > I'm trying to chew through the arch/arm/include/asm/pgtable-2level.h example to > see if I can make sense of it, I wonder if we can think about this slightly differently; READ_ONCE() has two important properties: - It guarrantees that a load will be issued, *even if output is unused* - It guarrantees that the read will be single-copy-atomic (no tearing) I think for the existing places where READ_ONCE() is used for pagetable reads we only care about: - It guarrantees that a load will be issued, *if output is used* - It guarrantees that the read will be single-copy-atomic (no tearing) I think if we can weaken to the "if output is used" property, then the compiler will optimize out all the unneccessary reads. AIUI, a C dereference provides neither of the guarrantees so that's no good. What about non-volatile asm? I'm told (thought need to verify) that for non-volatile asm, the compiler will emit it if the output is used and remove it otherwise. So if the asm contains the required single-copy-atomic, perhaps we are in business? So we would need a new READ_SCA() macro that could default to READ_ONCE() (which is stronger) and arches could opt in to providing a weaker asm version. Then the default pXdp_get() could be READ_SCA(). And this should work for all cases. I think. > >> >> Anyway. I am now designating you the expert at this ;) > > Oh no. :) > >> >>> >>>> >>>>> >>>>> IOW, push the READ_ONCE() down to the lowest level so the previous ones >>>>> (that will get essentially ignore?) will get folded into the last >>>>> READ_ONCE()? >>>>> >>>>> But my head still hurts and I am focusing on something else concurrently :) >>>> >>>> Even if we could make this work, I don't love that there's some implicit >>>> assumption there that could easily break later on. >>>> >>>> I'd rather we kept it as stupid/obvious as possible... >>> >>> Looking at include/asm-generic/pgtable-nopmd.h I am not sure we are talking >>> about implicit assumptions here. It's kind-of the design that the pud_t >>> values are dummies, so why shoul the pudp_get() give you any guarantees. >>> >>> At least that's my current understanding, which might be very flawed :) >> >> I mean I'm waving my hands around like I'm working on an aircraft carrier here >> so if you're _sure_ it's _absolutely_ safe then fine :) > > Well, not yet ... :) >