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 3B2AFC433FE for ; Mon, 31 Jan 2022 10:35:55 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id AFCB26B0073; Mon, 31 Jan 2022 05:35:54 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id A85786B0074; Mon, 31 Jan 2022 05:35:54 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 926606B0075; Mon, 31 Jan 2022 05:35:54 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (relay032.a.hostedemail.com [64.99.140.32]) by kanga.kvack.org (Postfix) with ESMTP id 807D66B0073 for ; Mon, 31 Jan 2022 05:35:54 -0500 (EST) Received: from smtpin01.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay13.hostedemail.com (Postfix) with ESMTP id 4ED4C60C4C for ; Mon, 31 Jan 2022 10:35:54 +0000 (UTC) X-FDA: 79090226628.01.C15637B Received: from mga06.intel.com (mga06.intel.com [134.134.136.31]) by imf02.hostedemail.com (Postfix) with ESMTP id 50C4780011 for ; Mon, 31 Jan 2022 10:35:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1643625353; x=1675161353; h=date:from:to:cc:subject:message-id:references: mime-version:in-reply-to; bh=YWtKq8OcN++bgZXQOD7vrGgwPikzV7AzgE9vpzi2/ms=; b=U00rZG9fr/mOFeWATyPZeD4fx4kfC2Ch6P6/i/fTwbwtmMVnpawEJGSp jCDWziWpHvsyCz4Cjvk9fmSRx8CYPX+d/gOHZ4wOz/nrcdH0yY8xH7imQ XT2P+6ECrgv8VmWw2gT5TIA7tNyEoXwLCmzOwJsqoR88eZz9wsDQzKUbi Z6ffieI0TA3eLyfcVPkuNBniNqECa41h+/P2xDu9DdIFU+G3q3fE7C74R O5Tmbb5OkRBkoTicJ+k5yckswGXE4PWi5MxZU2faldwwKFQYuC5gKthve 4aMhZko1cQeiXLZ1c67OBeMXPywZ85ylnVLKYwFbq1qhUjqTqMqxripdp g==; X-IronPort-AV: E=McAfee;i="6200,9189,10243"; a="308166354" X-IronPort-AV: E=Sophos;i="5.88,330,1635231600"; d="scan'208";a="308166354" Received: from fmsmga008.fm.intel.com ([10.253.24.58]) by orsmga104.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 31 Jan 2022 02:35:52 -0800 X-IronPort-AV: E=Sophos;i="5.88,330,1635231600"; d="scan'208";a="582605815" Received: from smile.fi.intel.com ([10.237.72.61]) by fmsmga008-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 31 Jan 2022 02:35:48 -0800 Received: from andy by smile.fi.intel.com with local (Exim 4.95) (envelope-from ) id 1nEU1E-00GpVR-3q; Mon, 31 Jan 2022 12:34:44 +0200 Date: Mon, 31 Jan 2022 12:34:43 +0200 From: Andy Shevchenko To: David Rientjes Cc: Waiman Long , Johannes Weiner , Michal Hocko , Vladimir Davydov , Andrew Morton , Petr Mladek , Steven Rostedt , Sergey Senozhatsky , Rasmus Villemoes , linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org, Ira Weiny , Rafael Aquini Subject: Re: [PATCH v2 1/3] lib/vsprintf: Avoid redundant work with 0 size Message-ID: References: <20220129205315.478628-1-longman@redhat.com> <20220129205315.478628-2-longman@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 50C4780011 X-Stat-Signature: agns873igks1zsoqpt4d5jdw8q64ye5u X-Rspam-User: nil Authentication-Results: imf02.hostedemail.com; dkim=pass header.d=intel.com header.s=Intel header.b=U00rZG9f; spf=none (imf02.hostedemail.com: domain of andriy.shevchenko@linux.intel.com has no SPF policy when checking 134.134.136.31) smtp.mailfrom=andriy.shevchenko@linux.intel.com; dmarc=pass (policy=none) header.from=intel.com X-HE-Tag: 1643625353-673809 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 Mon, Jan 31, 2022 at 12:30:33PM +0200, Andy Shevchenko wrote: > On Mon, Jan 31, 2022 at 12:25:09PM +0200, Andy Shevchenko wrote: > > On Sun, Jan 30, 2022 at 12:49:37PM -0800, David Rientjes wrote: > > > On Sat, 29 Jan 2022, Waiman Long wrote: > > > > > > > For *scnprintf(), vsnprintf() is always called even if the input size is > > > > 0. That is a waste of time, so just return 0 in this case. > > > > Why do you think it's not legit? > > I have to elaborate. > > For *nprintf() the size=0 is quite useful to have. > For *cnprintf() the size=0 makes less sense, but, if we read `man snprintf()`: > > The functions snprintf() and vsnprintf() do not write more than size bytes > (including the terminating null byte ('\0')). If the output was truncated due > to this limit, then the return value is the number of characters (excluding > the terminating null byte) which would have been written to the final string > if enough space had been available. Thus, a return value of size or more > means that the output was truncated. (See also below under NOTES.) > > If an output error is encountered, a negative value is returned. > > Note the last sentence there. You need to answer to it in the commit message > why your change is okay and it will show that you thought through all possible > scenarios. Also it seems currently the kernel documentation is not aligned with the code "If @size is == 0 the function returns 0." It should mention the (theoretical?) possibility of getting negative value, if vsnprintf() returns negative value. -- With Best Regards, Andy Shevchenko