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 B40271CEAA3; Thu, 27 Nov 2025 08:26:45 +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=1764232005; cv=none; b=Hl0nY6YFqjAIDuek3E3Aw8S+mbbUIN75oP6qoG6GNgdPRKKq4uvQLBRRL/jfsFvvB1U5sLz+PSfKf1YW9jx0FJz0fqyLsLPHt1AJ2hmmMip6DSruP+qC85eMRtT2Zw1UVINmEo7WQQeXr5eiAhNOJcvV6bwlfva7gAJFny2pk7k= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764232005; c=relaxed/simple; bh=bcPfE1QJFfuPJbcIojv7Nn8Gaq58C6i7WGhcnOmsWd4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=UHg1ODisU1bbTe8Lrncnl+JKcqTouRECAym8pJy1dWS6ptw5Lvq9lCMFyCPb9iJ4ON7T8KNJmViRtv1L7auItVrwe9DMRDaZu2o1Ty7E0YLFOLaD21qTEjfdt6dmUSBtZSYUNVgwweviYvSTqyqM6az8IUYwpJE0TDBQ/YLQZqU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=WcmFwl3B; 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="WcmFwl3B" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 336B6C4CEF8; Thu, 27 Nov 2025 08:26:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1764232005; bh=bcPfE1QJFfuPJbcIojv7Nn8Gaq58C6i7WGhcnOmsWd4=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=WcmFwl3BFSTa59/qQB3P8sjeCiQkh28Bx5X8iBQfaUe9YSkO9nM6cU7737TjnKOAp JJkTOlBk92QSIZ2v8m6Y6XbVK9HWVET1oCn6iBF1FjQogwNkbb80I07YujudKOOV/w jrIBVQvi9EmOYcwjbuxYPeKj4FIh8G+4H+CR5PI3EtQcfNOF6BBZM98kJDPYxtAGma 9DJFGkFyQVPcFXE1Z/hTrfVM345+WrNeKTmw1VLoc3DWXOvGUwLitd1sX0X02raP8b M5NVBmYkjvj75YhwFp8eFNQl9M+E9BJCXFBZubd8BB0leNXs9KcXf5d4SR52xj4DCC +M1Pg28PbX3VQ== Message-ID: Date: Thu, 27 Nov 2025 09:26:39 +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: "David Hildenbrand (Red Hat)" , 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: "Christophe Leroy (CS GROUP)" Content-Language: fr-FR In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Le 26/11/2025 à 15:22, Ryan Roberts a écrit : > 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://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flkml.kernel.org%2Fr%2F0019d675-ce3d-4a5c-89ed-f126c45145c9%40kernel.org&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C22d0a028b1ec4a8b678108de2cf73769%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C638997637481119954%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=ocR6usVgRHfue0MrtbQnDO8whINvy%2FDMAfNE3caiY8c%3D&reserved=0 >>>>>> >>>>>> 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). I think you can't assume pgdp_get() was already called, because some parts of code will directly descend to PMD level using pmd_off() or pmd_off_k() static inline pmd_t *pmd_off(struct mm_struct *mm, unsigned long va) { return pmd_offset(pud_offset(p4d_offset(pgd_offset(mm, va), va), va), va); } static inline pmd_t *pmd_off_k(unsigned long va) { return pmd_offset(pud_offset(p4d_offset(pgd_offset_k(va), va), va), va); } Christophe