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]) by smtp.lore.kernel.org (Postfix) with ESMTP id 41042C77B7A for ; Fri, 19 May 2023 15:30:38 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 88881900004; Fri, 19 May 2023 11:30:37 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 839BC900003; Fri, 19 May 2023 11:30:37 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 6D9C0900004; Fri, 19 May 2023 11:30:37 -0400 (EDT) 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 5DF5E900003 for ; Fri, 19 May 2023 11:30:37 -0400 (EDT) Received: from smtpin20.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay03.hostedemail.com (Postfix) with ESMTP id 66336A050E for ; Fri, 19 May 2023 15:30:34 +0000 (UTC) X-FDA: 80807391588.20.EF03BDC Received: from mail-wr1-f42.google.com (mail-wr1-f42.google.com [209.85.221.42]) by imf22.hostedemail.com (Postfix) with ESMTP id D0E02C001A for ; Fri, 19 May 2023 15:30:31 +0000 (UTC) Authentication-Results: imf22.hostedemail.com; dkim=pass header.d=gmail.com header.s=20221208 header.b="PyR/EY84"; spf=pass (imf22.hostedemail.com: domain of lstoakes@gmail.com designates 209.85.221.42 as permitted sender) smtp.mailfrom=lstoakes@gmail.com; dmarc=pass (policy=none) header.from=gmail.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1684510232; 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: in-reply-to:in-reply-to:references:references:dkim-signature; bh=TtZZ6uUexd434AXLFIEIjI9JV/0k8tvwBbLdbRsCUXo=; b=lB7j8SztcyONSnoBkelHVPWUsX8xUmLVNHg9anA7yqSVoquPUj4btMF8oQVBbSsOzd6Yd7 mFrxNfF6YP5iJnWdgRhIWlRhWc61hFb9JfPG7Ed+2F/34ALZDWjA5j8h0aP33tbCYGgEwU 4j1voWg/yNHNn/i8c0KmFaAWqVVQU9U= ARC-Authentication-Results: i=1; imf22.hostedemail.com; dkim=pass header.d=gmail.com header.s=20221208 header.b="PyR/EY84"; spf=pass (imf22.hostedemail.com: domain of lstoakes@gmail.com designates 209.85.221.42 as permitted sender) smtp.mailfrom=lstoakes@gmail.com; dmarc=pass (policy=none) header.from=gmail.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1684510232; a=rsa-sha256; cv=none; b=HVNfD2XiBiZbx1EifP4CYClWDpYozeX7kGLjxCxStdpUxJOyqVWRvGEMw8f4OJ0cJXXG1B bULxbiwdTj2gO+plCITVbLyYSePq90a5a24WfD0XjvnDcJAjBcYu9JpebQzm5rMrG+10xt 6T4RivSVYqhW1qdGXPBwnI2fbkZcv6c= Received: by mail-wr1-f42.google.com with SMTP id ffacd0b85a97d-3093a778089so2294450f8f.1 for ; Fri, 19 May 2023 08:30:31 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20221208; t=1684510230; x=1687102230; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:from:to:cc:subject:date:message-id:reply-to; bh=TtZZ6uUexd434AXLFIEIjI9JV/0k8tvwBbLdbRsCUXo=; b=PyR/EY842cKASD9G5w9Gq5/xB1KS97ac+lQU4cAzAlxoj7KZlKDqCPwqmHMjeOMOrv qcPIMvBWfeGMzdQ6uqpPs7Ovr6H/QpVogtdEuAJeR8EEDuSP00E0zPQmt+sDz45MbHoG h0kGWCHRaScH8OnixMhUfOPLGNy5N+Fm/6k6tUuqAETTdmhapR1JzXIvcJQN8IqjsfhD BN05ocwmxjqhDaTG9fFNA17KoNHz6wltqe3eoEvpHuBZFioRz9/9h7TdSrdIEGrh4FgZ OrNhmOZJPAf5ov1mPsV22peVwn5Z9VU8rEUKfSCFrhom7AcG9I8uL5U5v0cGS77oxsvB Ejfg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1684510230; x=1687102230; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=TtZZ6uUexd434AXLFIEIjI9JV/0k8tvwBbLdbRsCUXo=; b=bR6Am4VmnrA9fjrbhyH1WhsdHCiDQLOCYmbDPTbXBXd+qdWFyEDZyPydINg3l1AtTI A+STGMhGr722amkK0CzPn4iR5x20TKAeM9UL6nVS73LbxdSmn10MXeWs8r/a2pYIVGxl Eh5963PD+1u/rSWXfpAiY+jDS22vutG5HXA/EdaPjqnyH3eTAXRKPqMT39LvtUuACVHe AUTupaXATUedpAgcHkNq14StGz0Raq9uzzAz6o7WRgvGeC1Rj15sNCF51Kmr1q8ovXq8 fStzXBjhYz1Q3smkwlVLvRuDl6vPVb8NWlFSmop58nUQT/IQ2zHHrVDJNK37I+emrIwJ lkRw== X-Gm-Message-State: AC+VfDwK9oVODCVjcsN6se6ExUFRXzEXmPGsDHbDD10JoSWBO7AvAljZ eR1dyyLPP7yGfF7/bLsZwB0= X-Google-Smtp-Source: ACHHUZ52sgG6ZH6RefRvJGJis8biM2vI62TUrGHb64k5WVVmM+Y/pWEsCgexr9KGJ5ZJFH8IZVILjQ== X-Received: by 2002:a5d:544c:0:b0:2f9:61d4:1183 with SMTP id w12-20020a5d544c000000b002f961d41183mr2017687wrv.45.1684510229844; Fri, 19 May 2023 08:30:29 -0700 (PDT) Received: from localhost (host81-154-179-160.range81-154.btcentralplus.com. [81.154.179.160]) by smtp.gmail.com with ESMTPSA id y18-20020adff152000000b002feea065cc9sm5563195wro.111.2023.05.19.08.30.28 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 19 May 2023 08:30:28 -0700 (PDT) Date: Fri, 19 May 2023 16:30:27 +0100 From: Lorenzo Stoakes To: Arnd Bergmann Cc: Arnd Bergmann , Andrew Morton , Catalin Marinas , Will Deacon , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Ian Rogers , Adrian Hunter , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-perf-users@vger.kernel.org Subject: Re: [PATCH] [suggestion] mm/gup: avoid IS_ERR_OR_NULL Message-ID: <26e20ec3-51ec-43d0-8fdb-880e4343a77f@lucifer.local> References: <20230519093953.10972-1-arnd@kernel.org> <5b071f65-7f87-4a7b-a76a-f4a1c1568ae7@lucifer.local> <7d7ddc48-5985-4678-9f87-6e9b574a24d9@app.fastmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <7d7ddc48-5985-4678-9f87-6e9b574a24d9@app.fastmail.com> X-Rspamd-Queue-Id: D0E02C001A X-Rspam-User: X-Stat-Signature: 19neakzi8p7ef7gnsmybrh5ukexkxi31 X-Rspamd-Server: rspam01 X-HE-Tag: 1684510231-832374 X-HE-Meta: U2FsdGVkX1/KVRkpbSSLrch00+DT8lmQc+8NLhrYczTMOMokcFQfQQNebORIL5K/akWvc01Avkz/5ITb2yWmArVfk7foW++DIcKLyi6qam3iD5xoeQnM3PmCPWteKNxegV3EH//wEZbB1VAL0zxOmQ6UutAeNBGEeyiuCI9npNGYKY3yfK6HE3vYbWleadz4p9koOnugG5c2rpjnlFZwTHNCicHEJptnaWomFvpucpc2HkadYWBOmXU0zZ+uiGg3KJgGQg0rP5tykkxVQuMW7WL1G0jmfzXXwom/I3+QHp+bI+J654HeMpTKAgRB9lqhX7zghC8KqBagxupC3adudHvlTqfoopuJTI7bvVn5xk+Q0+nNOwjkvF3VBKFQukO3EncA9lJ9M7Zwm2q4iTuO2cRTOSuHxAlRDer8yZVcI5meC/dcZlqbqzyUSJcIR2twl3D5aEWOeHgz3GPIxaPydAUsJ3MXJz7zvWhwZITlGEcQ0Cr0O9Zks8Sz1aQaqDim/yJJGcSWIFNLteXK6n2f5g4W0I87NCXf4bMzmdaWp8IIfYbZz9to9Uzou918e1d9R1jNsK8YXwlJBZdVfRwHerXG40UCTWuVwu1mQQy5I0kMJrk2K+FjiPiuN2FCN5YWaNTqQxCpKb59QoXP0L8YU/CktdoOgFK5Cay7YpJHCqVScpZSpuqLHnK0FD5q0GGHCblljE6HqzJfuRdQQs/wLBUG4nkO8ZIzIBV9qaf+u6qGvnykMgTPudOHE0z/3ew5FtsacIWud8R0C4g5Uqd9N6+IYNKVtr/2GWQgkDZCyUYFUM/VEQammPeb2ISf6pmnddC4/fFMEibSA3ynAGnPiBFWiDyOiJh3tdrRUEEmQ0Ea2B46KvF1HuDYo8J5q9itBR9zXzRl60GnJxvRxFR6HvGG3YsGyLHQe3co+1IUYvYLqtfZYYMSDv9aAJ/IuU6APBP1MHXDpNfEm+8l+oY dHrn7rEF tQEFftD24wnGcHcE96GuaL/F75am5Ni6JA6HxnJTFNkntErlTm5UdqaEU5MYNX43fWK2AHZrJhmhHp+BHay3nAz/R8SpEzCRg0/d2AzBkQ5wYIlbH73V5MA5HszofX7z1R2SKgX2FkJ4pBCG92HyOYLVW7WMcvNjJi3LWNjglCeaYuBsz6JNMLCCcx6XXUiNDJYY+1zYpMumIYfcqmpJKmeufziA0lRqal8XepQDkfu82KozziMUH+/K3vZGWASBtlfT+cnwgH1kobXLIrcGRS7RxFKBr71FfVHVp6utcphdLaohuEKz64wHzyUjIydaZ0FC/Cn6TaaK7jhbrZHrZNErAtBqzicFiibE/BRCv/HgRVLLFpz9r0JzRAsNnyp9fi3IRBOkTe93UCABY0d2VeVe2wW4GupbOPc6YMhgxWVmTglkfRHmCmItVqw== 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: On Fri, May 19, 2023 at 05:09:35PM +0200, Arnd Bergmann wrote: > On Fri, May 19, 2023, at 16:51, Lorenzo Stoakes wrote: > > Given you are sharply criticising the code I authored here, is it too much > > to ask for you to cc- me, the author on commentaries like this? Thanks. > > My mistake, I expected this to get added automatically based on > the "Fixes:" tag, I probably dropped you by accident in the end. > OK no worries, it's often the way that something is purely accidental but seems ruder than intended (or even rude at all) because text is a terrible format :) > > On Fri, May 19, 2023 at 11:39:13AM +0200, Arnd Bergmann wrote: > >> From: Arnd Bergmann > >> > >> While looking at an unused-variable warning, I noticed a new interface coming > >> in that requires the use of IS_ERR_OR_NULL(), which tends to indicate bad > >> interface design and is usually surprising to users. > > > > I am not sure I understand your reasoning, why does it 'tend to indicate > > bad interface design'? You say that as if it is an obvious truth. Not > > obvious to me at all. > > > > There are 3 possible outcomes from the function - an error, the function > > failing to pin a page, or it succeeding in doing so. For some of the > > callers that results in an error, for others it is not an error. > > > > Overloading EIO on the assumption that gup will never, ever return this > > indicating an error seems to me a worse solution. > > The problem is that we have inconsistent error handling in functions > that return an object, about half of them use NULL to indicate an error, > and the other half use ERR_PTR(), and users frequently get those > wrong by picking the wrong one. Functions that can return both make > this worse because whichever of the two normal ways a user expects, > they still get it wrong. > > > Not a fan at all of this patch, it doesn't achieve anything useful, is in > > service of some theoretical improvement, and actually introduces a new > > class of bug (differentiating EIO and failing to pin). > > Having another -EIO return code is a problem, so I agree that > my patch wouldn't be good either. Maybe separating the error return > from the page pointer by passing a 'struct page **p' argument that > gets filled would help? Yeah I see your point, in the majority of cases failing to pin is an error, I just wonder if something like adding another parameter wouldn't just add more noise/confusion here than it saves? Sadly I think aspects of this are C sucking at dealing with multiple return values sanely, and there probably isn't a totally nice way of dealing with this. > > Arnd