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 A1D32326D6A; Wed, 26 Nov 2025 16:35:01 +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=1764174903; cv=none; b=BRoosIuKL4kuFgOjtFojC8Oj/7aAiPLmOZhni2Vl7+Sai0y4LaEcgkHESndAq1msW3dXoWgG5RCMikaA3PELBaI5EXNkvKEy9ebZTej7rWW9s8oHg4zRM6ooZ4zZ5LbRqSO9/vR4ZZDltn9k+jkRK4Uw4Xmf+R1uTSdoCHtvEko= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764174903; c=relaxed/simple; bh=gxAq95DCSNG2xRY6Jo4wxNF/hQkQq5IKfxEJoPNewi4=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=bWgi3oxOBqbZZctwpKXieQQM041hqAHcGYmjZONybqkH35aNZBkpk3XC+9bhHkdeftpEtlYqZ7UXyNUcSZGPia/d2oCtwKdq+fpncYWo8yUPx35bje6ctgecxFHOXTlzV3gKSReCevzq2hm+1bJP8BCrrsFa6MCm/f3p2ouS21w= 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 0459C168F; Wed, 26 Nov 2025 08:34:48 -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 B2E773F73B; Wed, 26 Nov 2025 08:34:52 -0800 (PST) Message-ID: <150ffcb7-2df2-4f3a-a12e-9807f13c6ab9@arm.com> Date: Wed, 26 Nov 2025 16:34:51 +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 From: Ryan Roberts 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> <4505a93b-2bac-4ce1-8971-4c31f1ce1362@arm.com> In-Reply-To: <4505a93b-2bac-4ce1-8971-4c31f1ce1362@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 26/11/2025 16:07, Ryan Roberts wrote: > 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. I'm not sure this works. It looks like the compiler is free to move non-volatile asm sections which might be problematic for places where we are currently using READ_ONCE() in lockless algorithms, (e.g. GUP?). We wouldn't want to end up with a stale value. Another idea: Given the main pattern where we are aiming to optimize out the read is something like: if (!pud_present(*pud)) where for a folded pmd: static inline int pud_present(pud_t pud) { return 1; } And we will change it to this: if (!pud_present(pudp_get(pud))) ... perhaps we can just define the folded pXd_present(), pXd_none(), pXd_bad(), pXd_user() and pXd_leaf() as macros: #define pud_present(pud) 1 Then the compiler will never even see the pudp_get(). Thanks, Ryan > >> >>> >>> 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 ... :) >> >