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 F2E4B302755; Wed, 26 Nov 2025 14:46:47 +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=1764168408; cv=none; b=MTYmWA/4pCA0SINXOZ8PaShxuycja5neREkORcPlVDXCJYQIWhzK4R9lDnCrRDhEwTc7fsWnayuuZ1PTfiw1ukJPH49ARg9NToze7OIpSIHGJSSNclwSiM/hNsF5YtsvOvYXKtzCnGScWEfAiW87VxVqYTU3lEsxbKCAner9LsQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764168408; c=relaxed/simple; bh=rN8uuyU8RJxsQK9SI2vsik5bVZRYdR4fDfMryS9LdTo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=j0HU3+gP9i0BFA6dwIPNpNBsaJDUolYgU+xglKrtd6wugSQCbZL4WxvxU8/7tJ2bRJ0KGnfqKUZ1tKIJkp2HvdIaFpljX0hPSCL8XF1o8I8KJRXpDVN5BDb9R1QOLQtg7u5bPeCR9nlYR/tlFjhN+D1e4FseEtJmDvT8SK3Vzjk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Knb6dB6K; 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="Knb6dB6K" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D951FC4CEF7; Wed, 26 Nov 2025 14:46:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1764168407; bh=rN8uuyU8RJxsQK9SI2vsik5bVZRYdR4fDfMryS9LdTo=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=Knb6dB6KKmlnmfJpGMk6rLZZg/Xct6yBU+L1A5tJGCzb90ZgbFIwltq2rnJrqAlb7 1m5cTDHSno9KWGcw9J9xR1Vx6XYBkWdCJzainHhBbiuiOXeYZyf1CNuUDvRsEFpkVV 7IJu0+1HHJuTC+uWYmKLKfGHbKpiReXDB7NMDr5kMlBx9faWmr3mgZ6PD6RzEcGh6y wtbFL4cEKyAzFnXjBZeg4brZpmGf/C3fwJost64KZLAqrMU0dQsh+XijAoVcsQU8Ij 3VZVb7gospy7DoVSKnuqOIyjQ0D9N3e2LKLO3PtlK7etI21gf96UYXggvB3TqwbADK adsHLAl4WA45A== Message-ID: <6b966403-91e0-4f06-86a9-a4f7780b9557@kernel.org> Date: Wed, 26 Nov 2025 15:46:40 +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 , Wei Yang Cc: Lorenzo Stoakes , 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 15:22, 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. Yeah, I'm also still trying to understand how it could work. > > 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). With __PAGETABLE_PMD_FOLDED we treat the PUD to be fake-present, like static inline int pud_present(pud_t pud) { return 1; } And obtaining the pmd_t* is essentially cast of the pud_t* static inline pmd_t * pmd_offset(pud_t * pud, unsigned long address) { return (pmd_t *)pud; } So in that case we might want to have the READ_ONCE() remove from the pudp_get(), not the pmdp_get()? 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 :) -- Cheers David