From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4EF4B285C9F; Thu, 27 Nov 2025 19:39:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764272352; cv=none; b=OBmnw+K0oYaKjy5SYTJJ5dTt0EQotCPFDlsqsROFPk9sC7K1djPVWK6KjiWcM30t8DrYwC/ydYwvPOSPQDyyFwg+S5ewJyHVVDwxs/Q9FEUt+H8rV+f9C5QeHKZF5jI8S4FkvVKyPE0fDEMFTnc5hKgNA/iNUtQJ2ZEPeYsZ10U= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764272352; c=relaxed/simple; bh=9CmP20l5fh1wI0Qc9aslBQ2/q51oJ57cp2te1agYTJ4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Oytfz1/mA81lY17a4rJABzCcZjEaPGuSePj0L+Fme5a2YzL2IWvoxM18lTJpW7wYw2Fuykfd615EZsl6Zp9mFAKb2f9b3+FS2FanAfYUUbGqrHhI7R2s5cHYfpHnX5W5mxX7EejyZWicpcCqetDvasLNRWBxsIHzQT9xwaKRKjE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=CC1qyyXK; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="CC1qyyXK" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 671B7C4CEF8; Thu, 27 Nov 2025 19:39:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1764272350; bh=9CmP20l5fh1wI0Qc9aslBQ2/q51oJ57cp2te1agYTJ4=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=CC1qyyXKyo4HeL/hyEiAmjyrdFYZCtsA9ufEs1ZbODnUMjPMpmfB0ekb5LgdLYCLX CHpUVRLmWiP618uKzGcw7wbVXFiwRW6iCG11whNx4s8RBhJ8T9/2zgzHzeMA5neH+X jzNpS88WZd5RjEB1yebsUUYBbWhpHDbiEFxPLMdhwQ4oBHPrIE4ZWkloGCINaM2Td4 NRFHi45iMDD6psQhkBxKT8Hte/o1Jf+XR9XXzKyaG1hLm6nKuFwYsMDrEhw0+17Fso Ig8qddeXGqDngyGmHRXeegLObMvmeRhAeq0whuZCSxPlUukv5cYyvoYlWUMy+FQ+1D EZxUxaIVmEl9w== Message-ID: Date: Thu, 27 Nov 2025 20:39:01 +0100 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 To: Ryan Roberts , "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> <150ffcb7-2df2-4f3a-a12e-9807f13c6ab9@arm.com> From: "Christophe Leroy (CS GROUP)" Content-Language: fr-FR In-Reply-To: <150ffcb7-2df2-4f3a-a12e-9807f13c6ab9@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Le 26/11/2025 à 17:34, Ryan Roberts a écrit : > 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(). Take care about macros with unused parameters, it can be a problem sometimes, see commit 05a4ab823983 ("powerpc/uaccess: fix warning/error with access_ok()") Christophe