From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 5E810CFD2F6 for ; Thu, 27 Nov 2025 08:35:50 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id AD9A96B0022; Thu, 27 Nov 2025 03:35:49 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id AB1206B0023; Thu, 27 Nov 2025 03:35:49 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 9ED5C6B0024; Thu, 27 Nov 2025 03:35:49 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0011.hostedemail.com [216.40.44.11]) by kanga.kvack.org (Postfix) with ESMTP id 8CEDB6B0022 for ; Thu, 27 Nov 2025 03:35:49 -0500 (EST) Received: from smtpin24.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay08.hostedemail.com (Postfix) with ESMTP id 2687B140989 for ; Thu, 27 Nov 2025 08:35:49 +0000 (UTC) X-FDA: 84155728818.24.C0E54F9 Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf03.hostedemail.com (Postfix) with ESMTP id 505852000A for ; Thu, 27 Nov 2025 08:35:47 +0000 (UTC) Authentication-Results: imf03.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=JgZwtp8T; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf03.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1764232547; a=rsa-sha256; cv=none; b=pnYOJvEi/YqsMynVGivLplP43Lp0olZiJ6MUW3Y8ZYO3VmjfapSdMT+k88KZe+PojRJKhQ kG+n1MOMApNz3P3V+1UWUhb77D3cVh49MkkcFdLSiYxTqh3lWOV0Z4+51NUBuou0/Y1Vob HiXfqW4HlvXAom5uAUrKlk76AYu9L0c= ARC-Authentication-Results: i=1; imf03.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=JgZwtp8T; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf03.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1764232547; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=NN/qtxM9gQI/WFD3wEaliU/Ai+7zohP2JGTV+gE/dOA=; b=W0/VcRCK+YBNzBzvL9JBa9DyIQ2I3jF93tZTYdYML3t5obei0SuskRSPX9e0EbBLifbSdC lyiHDeZlArq591NIgpSXxMPdJ67XOlRJ8h+/YB/za6tz4E7WMRbwubwH8AR8U+fd5Ox9dL OpCCUpjH8MtWCui99QVrW1jhiiT+guI= Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id 3167540879; Thu, 27 Nov 2025 08:35:46 +0000 (UTC) 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 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 X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 505852000A X-Stat-Signature: btnkdiouekdu4eu19hkb1emsykztc4fi X-Rspam-User: X-HE-Tag: 1764232547-682794 X-HE-Meta: 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 9fvrGh3X VHmd9EzGuED5ElEXxFv0xZYCS0+SvCCxcz0nahsgZ+7L+y5J3fTYYUF6mGXiya/qFVkghHOEAXNIFMKXSQ+9U1wPs9udLrceoWZSe2bmUQuaF++FzF/fhrpqMuprVtA596E/xKx1ZHUePhxlaZkfGks4HcBVaZlRL/IDBc1Hbb5R97s+MxWn7Ixhx451ZqKqd3Qtab87Lx+GDHIueXruAirj5Z8B0FqsFKPLv7Fvf8mtZhQcRbcd0dyW/AdT6KusnV6vVkRLt8fThJCSqpwIU5b4ufuKTxstPV7DpiIa+nUkBqFEesZPbVqt7NMN8K00GlyUKJECUTpNugKqAk6fa+1aTBy0YHRe2MGP4LxiMhCKGhaUmr++WS0LSq9BiVaPSAiqFxfyL5VP6xAUk7EdYSrQ9gs8iq3dHLN5tkbAyq9ofiX+CFA+iVaJ7NkCUEG7Mj0bihGzI9/hH8rM= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 0ACB0D11183 for ; Thu, 27 Nov 2025 08:36:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:Content-Type: Content-Transfer-Encoding:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:From:References:Cc:To:Subject: MIME-Version:Date:Message-ID:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=i9diL5mbibq9YBRdQ1heQBENsOErJJ9pTufADa3BfcI=; b=GOEEPLQ5YxOw7u w63qQIEsfes/jz84dS0mEq+0vz27DM5SDS1DqcmntvX5NzyoJvoglIm0Wo7PMIvar1HTPb2jpCAst 0YIfhNqXlNg+1JZqO6D/PQfFnSzCYAtHdEFtPmGNdjxKHqFG9LJdksXQTs6sfAlEIIxrn/YSrJ/p9 CaUPiosClc2g0WQ5zxA2gTwTGTmH5y2TET8cBkV2A6MaiPOV6umgT10znnM5tIIBhKQ1avfyrDMaX OLT3EN+gnPoiaQTUPmn1B3W5vaMvdM9jBdPEe5OVHxBZvN+0ql6FrZ8njMd633VBOAslTFbi9dl1O ZrSYrIcG28spu92VCPSA==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vOXTh-0000000GDGG-3dU0; Thu, 27 Nov 2025 08:35:49 +0000 Received: from sea.source.kernel.org ([2600:3c0a:e001:78e:0:1991:8:25]) by bombadil.infradead.org with esmtps (Exim 4.98.2 #2 (Red Hat Linux)) id 1vOXTf-0000000GDFs-0RDX for linux-riscv@lists.infradead.org; Thu, 27 Nov 2025 08:35:48 +0000 Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id 3167540879; Thu, 27 Nov 2025 08:35:46 +0000 (UTC) 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 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: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20251127_003547_206841_DAE84F09 X-CRM114-Status: GOOD ( 19.99 ) X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Transfer-Encoding: base64 Content-Type: text/plain; charset="utf-8"; Format="flowed" Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org 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