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 B11B5302CB3; Wed, 26 Nov 2025 14:53:37 +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=1764168817; cv=none; b=cOAStoJnQH8k6ME4SBBUyfxt7d7W4ZxwvfCxAbtVEhifrrp28wta+uUQgCV75xSSIBjWLanwnqhuW9CGjS3Nd8+rRbDNjnZ6n4wtDZVwzk08MppIiuP4JJH6VxcLMPKNJEXrUw0/YnMSzawZDjQQHlhwp/Z0rVpu2XwJn6C+BOM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764168817; c=relaxed/simple; bh=c3XYY0cYtWPbzdfFG05mXSc/WqtZze6j0CNfdBzcecg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=HgnC7ADUc29k8JTZbOoXGOtv6qNT6WK3G2FaHEdmsXC3Dc55uRA08AOvs/nQG8hsk8pyVPkEiuSmDkuNrKK6Wi2n/bgimSpbpnhTi5q7K4T51OgBxgSJjfvKCAQqnHsHZdNJ4aFBIYgf0cwuVJl5zMw+z/f9W6HAYEyVX2vDdMo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=iKA8ZPZB; 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="iKA8ZPZB" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 22155C4CEF7; Wed, 26 Nov 2025 14:53:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1764168817; bh=c3XYY0cYtWPbzdfFG05mXSc/WqtZze6j0CNfdBzcecg=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=iKA8ZPZBzJCQYVEC7nRsRPwzuuWCfBhocWIEE6hhn+ym5dSpRSL0fAPhaA+Clc/qg Acxjig2SijB4lbZWCCfVV+gD5keP+RT0r+pRsypmohhmQS1poaaqmC0ozypxxymmEG bFNhTQliyy/QUx2BoZ6GnpplECtgUZz1k8uSqlQueSYJrbJTZWEnezPFBR2iicpBNv VvQdvJLq66DJ3TDKqr1po8RlbDflk5NTEaXIGwhIEd8GA3vlrtFeGRlnoNqkHtKQZy ZYtNKHlp5O9GEwLFOR33JAkEXGpVE81UDAWVtj/1iiW2t2+3VFj6LQeI7m054cCRrR LWQawhT0xSlGA== Message-ID: <82dca16e-6f71-45a9-9748-db47c1f42597@kernel.org> Date: Wed, 26 Nov 2025 15:53:30 +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: Lorenzo Stoakes , Ryan Roberts 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: <20251113014656.2605447-1-samuel.holland@sifive.com> <20251113014656.2605447-7-samuel.holland@sifive.com> <02e3b3bd-ae6a-4db4-b4a1-8cbc1bc0a1c8@arm.com> <6bdf2b89-7768-4b90-b5e7-ff174196ea7b@lucifer.local> <71123d7a-641b-41df-b959-88e6c2a3a441@kernel.org> <20251126134726.yrya5xxayfcde3kl@master> From: "David Hildenbrand (Red Hat)" Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 11/26/25 15:37, Lorenzo Stoakes wrote: > On Wed, Nov 26, 2025 at 02:22:13PM +0000, Ryan Roberts wrote: >> On 26/11/2025 13:47, Wei Yang wrote: >>> On Wed, Nov 26, 2025 at 01:03:42PM +0000, Ryan Roberts wrote: >>>> On 26/11/2025 12:35, David Hildenbrand (Red Hat) wrote: >>> [...] >>>>>>>>> Hi, >>>>>>>>> >>>>>>>>> I've just come across this patch and wanted to mention that we could also >>>>>>>>> benefit from this improved absraction for some features we are looking at for >>>>>>>>> arm64. As you mention, Anshuman had a go but hit some roadblocks. >>>>>>>>> >>>>>>>>> The main issue is that the compiler was unable to optimize away the >>>>>>>>> READ_ONCE()s >>>>>>>>> for the case where certain levels of the pgtable are folded. But it can >>>>>>>>> optimize >>>>>>>>> the plain C dereferences. There were complaints the the generated code for arm >>>>>>>>> (32) and powerpc was significantly impacted due to having many more >>>>>>>>> (redundant) >>>>>>>>> loads. >>>>>>>>> >>>>>>>> >>>>>>>> We do have mm_pmd_folded()/p4d_folded() etc, could that help to sort >>>>>>>> this out internally? >>>>>>>> >>>>>>> >>>>>>> Just stumbled over the reply from Christope: >>>>>>> >>>>>>> https://lkml.kernel.org/r/0019d675-ce3d-4a5c-89ed-f126c45145c9@kernel.org >>>>>>> >>>>>>> And wonder if we could handle that somehow directly in the pgdp_get() etc. >>>> >>>> I certainly don't like the suggestion of doing the is_folded() test outside the >>>> helper, but if we can push that logic down into pXdp_get() that would be pretty >>>> neat. Anshuman and I did briefly play with the idea of doing a C dereference if >>>> the level is folded and a READ_ONCE() otherwise, all inside each pXdp_get() >>>> helper. Although we never proved it to be correct. I struggle with the model for >>>> folding. Do you want to optimize out all-but-the-highest level's access or >>>> all-but-the-lowest level's access? Makes my head hurt... >>>> >>>> >>> >>> You mean sth like: >>> >>> static inline pmd_t pmdp_get(pmd_t *pmdp) >>> { >>> #ifdef __PAGETABLE_PMD_FOLDED >>> return *pmdp; >>> #else >>> return READ_ONCE(*pmdp); >>> #endif >>> } >> >> Yes. But I'm not convinced it's correct. >> >> I *think* (but please correct me if I'm wrong) if the PMD is folded, the PUD and >> P4D must also be folded, and you effectively have a 2 level pgtable consisting >> of the PGD table and the PTE table. p4dp_get(), pudp_get() and pmdp_get() are >> all effectively duplicating the load of the pgd entry? So assuming pgdp_get() >> was already called and used READ_ONCE(), you might hope the compiler will just >> drop the other loads and just use the value returned by READ_ONCE(). But I doubt >> there is any guarantee of that and you might be in a situation where pgdp_get() >> never even got called (perhaps you already have the pmd pointer). > > Yeah, it kinda sucks to bake that assumption in too even if we can prove it > currently _is_ correct, and it becomes tricky because to somebody observing this > they might well think 'oh so we don't need to think about tearing here' but in > reality we are just assuming somebody already thought about it for us :) Looking at include/asm-generic/pgtable-nopmd.h, PUD entries there are * always present (pud_present() == 1) * always a page table (pud_leaf() == 0) And pmd_offset() is just a typecast. So I wonder if that means that we can make pudp_get() be a simple load (!READ_ONCE) because nobody should possibly do something with that value as we must perform the pmd_present() checks etc. later and obtain the PMD through a READ_ONCE(). So far my thinking, maybe it's flawed :) -- Cheers David