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 8B9DBD10F58 for ; Wed, 26 Nov 2025 16:07:32 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id CF9F86B0026; Wed, 26 Nov 2025 11:07:31 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id CD1D66B007B; Wed, 26 Nov 2025 11:07:31 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id BE73A6B0089; Wed, 26 Nov 2025 11:07:31 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id A8EDD6B0026 for ; Wed, 26 Nov 2025 11:07:31 -0500 (EST) Received: from smtpin28.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay07.hostedemail.com (Postfix) with ESMTP id 532A21606C4 for ; Wed, 26 Nov 2025 16:07:31 +0000 (UTC) X-FDA: 84153238302.28.1AEDCD6 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf19.hostedemail.com (Postfix) with ESMTP id E22F31A000F for ; Wed, 26 Nov 2025 16:07:28 +0000 (UTC) Authentication-Results: imf19.hostedemail.com; dkim=none; dmarc=pass (policy=none) header.from=arm.com; spf=pass (imf19.hostedemail.com: domain of ryan.roberts@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=ryan.roberts@arm.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1764173249; a=rsa-sha256; cv=none; b=B6Yh0/FpVc1FfhVrLSk8WWxAJDy0vugZw7cU9nvaXOdhSEz/ok+K9u5OjQLZ8Gm6bl5UWX INkI5lCWTdmF4y6hQldzsq5zG17t4x2AlSBnisUyqBhWRV8wAA2Ru6gqlIFfqJcdt++HTn ohzuswbPaEbHD9oL2MNe7Ae3qLlmMoI= ARC-Authentication-Results: i=1; imf19.hostedemail.com; dkim=none; dmarc=pass (policy=none) header.from=arm.com; spf=pass (imf19.hostedemail.com: domain of ryan.roberts@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=ryan.roberts@arm.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1764173249; 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; bh=+RW7cdtseao8/7aTNgdH2Rhr+bJ6eoSju9CyIpiM7Cw=; b=Ztf50RsVTL05xTFu1BtmbNelkSZwOsY0C4+je62WoLvy7KXBTJFZhRqwnaa19fTXclMReR yI9HalOA76N9xlCEgF+uyGShjC+ILOF1vwSsExMsxiY9LVAiHPBPj7TSSats74BlEj7xX7 pPxguRw/+S+xCncF7lOAhLidKNLxpg0= Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 4B9C41692; Wed, 26 Nov 2025 08:07:20 -0800 (PST) Received: from [10.1.33.153] (unknown [10.1.33.153]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 051ED3F73B; Wed, 26 Nov 2025 08:07:24 -0800 (PST) Message-ID: <4505a93b-2bac-4ce1-8971-4c31f1ce1362@arm.com> Date: Wed, 26 Nov 2025 16:07:23 +0000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 06/22] mm: Always use page table accessor functions Content-Language: en-GB To: "David Hildenbrand (Red Hat)" , Lorenzo Stoakes 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: <6bdf2b89-7768-4b90-b5e7-ff174196ea7b@lucifer.local> <71123d7a-641b-41df-b959-88e6c2a3a441@kernel.org> <20251126134726.yrya5xxayfcde3kl@master> <6b966403-91e0-4f06-86a9-a4f7780b9557@kernel.org> <1ca9f99f-6266-47ca-8c94-1a9b9aaa717f@kernel.org> <37973e21-e8f4-4603-b93d-4e0b1b2499fa@lucifer.local> From: Ryan Roberts In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: E22F31A000F X-Stat-Signature: 5idqjiryyd7zqeyooxybs5chbkks9s8c X-Rspam-User: X-HE-Tag: 1764173248-541596 X-HE-Meta: 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 FSRgu9jI cq81x8MpM/ezvDj3Jh3/sAxfSLo1Ag5AJD9pCq8jd5IQsTvX0La2wZITg9Wv2Bb8D1ygtOksTAfoZvoLvPQCh6DC8Czv8ax264t+1CEkIUxwIM3imGV5jkRuVJvTb2pSkPAAN0xRSBohcFR0f/ZmB4ELR86n/ui5ZsxwGqTrOhf1mH/Hys0IuBEmW+Q== 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 26/11/2025 15:12, David Hildenbrand (Red Hat) wrote: > On 11/26/25 16:08, Lorenzo Stoakes wrote: >> On Wed, Nov 26, 2025 at 03:56:13PM +0100, David Hildenbrand (Red Hat) wrote: >>> On 11/26/25 15:52, Lorenzo Stoakes wrote: >>>> >>>> Would the pmdp_get() never get invoked then? Or otherwise wouldn't that end up >>>> requiring a READ_ONCE() further up the stack? >>> >>> See my other reply, I think the pmdp_get() is required because all pud_* >>> functions are just simple stubs. >> >> OK, thought you were saying we should push further down the stack? Or up >> depending on how you view these things :P as in READ_ONCE at leaf? > > I think at leaf because I think the previous ones should essentially be only > used by stubs. > > But I haven't fully digested how this is all working. Or supposed to work. > > I'm trying to chew through the arch/arm/include/asm/pgtable-2level.h example to > see if I can make sense of it, I wonder if we can think about this slightly differently; READ_ONCE() has two important properties: - It guarrantees that a load will be issued, *even if output is unused* - It guarrantees that the read will be single-copy-atomic (no tearing) I think for the existing places where READ_ONCE() is used for pagetable reads we only care about: - It guarrantees that a load will be issued, *if output is used* - It guarrantees that the read will be single-copy-atomic (no tearing) I think if we can weaken to the "if output is used" property, then the compiler will optimize out all the unneccessary reads. AIUI, a C dereference provides neither of the guarrantees so that's no good. What about non-volatile asm? I'm told (thought need to verify) that for non-volatile asm, the compiler will emit it if the output is used and remove it otherwise. So if the asm contains the required single-copy-atomic, perhaps we are in business? So we would need a new READ_SCA() macro that could default to READ_ONCE() (which is stronger) and arches could opt in to providing a weaker asm version. Then the default pXdp_get() could be READ_SCA(). And this should work for all cases. I think. > >> >> Anyway. I am now designating you the expert at this ;) > > Oh no. :) > >> >>> >>>> >>>>> >>>>> 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 :) >>>> >>>> Even if we could make this work, I don't love that there's some implicit >>>> assumption there that could easily break later on. >>>> >>>> I'd rather we kept it as stupid/obvious as possible... >>> >>> Looking at include/asm-generic/pgtable-nopmd.h I am not sure we are talking >>> about implicit assumptions here. It's kind-of the design that the pud_t >>> values are dummies, so why shoul the pudp_get() give you any guarantees. >>> >>> At least that's my current understanding, which might be very flawed :) >> >> I mean I'm waving my hands around like I'm working on an aircraft carrier here >> so if you're _sure_ it's _absolutely_ safe then fine :) > > Well, not yet ... :) >