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 84CE527F727; Thu, 27 Nov 2025 08:35:46 +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=1764232546; cv=none; b=eodPhSDLd7B+MW7IC0uUJiEp5Cjs+WrlCvmqi/WJrZEexqhlRLFzZ+wVmYj1rmQ9a+lPM2VAVf9teM8IBBcJuOLAXb1wWrf1kqt10qmmKuUMSLqY4xczWm2Oo1tCV09SmK+fjjub6QiH51JNXUv1K6FPJ0LkxPv9gu2uCqwcUq4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764232546; c=relaxed/simple; bh=qx0VZrdqJksxpKXVCVyXb4JAI2P5dCq0KsV9HbIKGek=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ltg3X7YddB6BvKa+Dqfv77AW9TRATPWYStLL35R0XR2mtusFlH00m2r9dB4NL/sL0i9ilrQlCU9MHLUw/W3j0HNXGeBrF2+Q8OGcSuKuXMq7a+ZGiORouCGHmg1LnQ9lLGfzgu4mBkQfuCrq9ut++PkIgN3uPKCrwm2PwJ42tbQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=JgZwtp8T; 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="JgZwtp8T" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9E346C4CEF8; Thu, 27 Nov 2025 08:35:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1764232546; bh=qx0VZrdqJksxpKXVCVyXb4JAI2P5dCq0KsV9HbIKGek=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=JgZwtp8Tu+n8p3LaI0AsCxvsyQZRHcTfkmQ3l9GMb6+DsJHERvHrUAVvuNANZGssM 1M16aKThUv8pm2fpPqzK2E3IHdPvq+aq3wozuehRIzN/auBNvkuBrLr6jWjsAJGCMq gKw7f/dbAyg/ijfp+HAJRn+vODfnzRDqa1NPE8PNzovArQpUEyQnNO9IlK9lVOFcaU rIQPl3NMUd1o23B2Hqadw35UJ/5HrnWAquZp5u0emedD51lI/HoUp9yu7BgsLOvQ8k XMwxnK1OciERhD8X2cpmZm/yFv7fG7M0DIasL3/JLatgVzgnVpA/FBj2y3iK0eiQu3 ClsmI9yWdvtIg== Message-ID: <46ca641c-6a39-49e4-b1b2-23262841ecc8@kernel.org> Date: Thu, 27 Nov 2025 09:35:37 +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: "Christophe Leroy (CS GROUP)" , 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: 8bit On 11/27/25 09:26, Christophe Leroy (CS GROUP) wrote: > > > 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); > } I'll note that these (nesty) helpers only work when you know that you have folded page tables. And that's why I am arguing that the pmdp_get() must actually be kept as is. -- Cheers David