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 97748CD4F3C for ; Mon, 18 May 2026 12:57:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=BmJ9Xymxb1grPg7JOpGoiTsi7Q3hTKH2JwMsAoVhd1o=; b=lY/jmAKKmIgmupq9j5lmTpZQlh NnmiPz9jl8fpRMd5rQEa75d9T4mryEu51NMTtruFk6fh2P9jtgBBWzi64srNpm03kmitNw2rPLOfx WcGeEfcYgkKL8pMrc72RHa7vuFOANNyqQY9pa9e6sIhKnxQf0LUT8Fa7GKiJzSuSmDq9Xbt1QOV9p P7kC1S6oQVYzZwjy5C8SyZt5hm61rx0Ryd53nx32S7Zldo3zqRFxjhzTQW2F3UD067xmDUwbWy/eX eRHDL5vfVgLzcdT3IqtGLEPn1c/zIZ3/zEjpYzjftXyOaZJy08HtKLwgmbBgQgCOXQypaR2FXbrH5 stDhAE7A==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1wOxX6-0000000FjFL-3tvk; Mon, 18 May 2026 12:57:20 +0000 Received: from verein.lst.de ([213.95.11.211]) by bombadil.infradead.org with esmtps (Exim 4.99.1 #2 (Red Hat Linux)) id 1wOxX4-0000000FjDl-1tDr for linux-nvme@lists.infradead.org; Mon, 18 May 2026 12:57:19 +0000 Received: by verein.lst.de (Postfix, from userid 2407) id E839E68D05; Mon, 18 May 2026 14:57:13 +0200 (CEST) Date: Mon, 18 May 2026 14:57:13 +0200 From: Christoph Hellwig To: Pavel Begunkov Cc: Christian =?iso-8859-1?Q?K=F6nig?= , Christoph Hellwig , Jens Axboe , Keith Busch , Sagi Grimberg , Alexander Viro , Christian Brauner , Andrew Morton , Sumit Semwal , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org, linux-fsdevel@vger.kernel.org, io-uring@vger.kernel.org, linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org, Nitesh Shetty , Kanchan Joshi , Anuj Gupta , Tushar Gohad , William Power , Phil Cayton , Jason Gunthorpe Subject: Re: [PATCH v3 04/10] block: introduce dma map backed bio type Message-ID: <20260518125713.GC5754@lst.de> References: <646ecd6fde8d9e146cb051efb514deb27ce3883e.1777475843.git.asml.silence@gmail.com> <20260513081929.GD5477@lst.de> <24833f76-2289-4859-86d1-9215b11a1258@gmail.com> <4561c621-817c-46be-8ff0-0b557f6c819d@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4561c621-817c-46be-8ff0-0b557f6c819d@gmail.com> User-Agent: Mutt/1.5.17 (2007-11-01) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260518_055718_652638_D8C80FA0 X-CRM114-Status: GOOD ( 21.35 ) X-BeenThere: linux-nvme@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "Linux-nvme" Errors-To: linux-nvme-bounces+linux-nvme=archiver.kernel.org@lists.infradead.org On Mon, May 18, 2026 at 01:40:18PM +0100, Pavel Begunkov wrote: >> When that is really a performance critical path then you can use the likely() and unlikely() macros to give the compiler the hint which one to prefer. > > That might be more penalising than placing them in the right order, > and it might be fine as it's new and all that, but it's not a clear > cut as it's definitely not created to be a slow path. Yes. Whatever the caller is using at a given time is the fast path here, and dynamic branch prediction in modern cpus handles this really well. > TBH, not sure > why we're bike shedding such things, is it somewhere in the code > style? It makes reading the code annoying, so it better have a good reason. Now for a single conditional it's not much of an issue, but these things tend to pile up and then start to get really annoying. Always write your code the most straight forward way unless you have a good reason not to. >> What could be useful is to have the true path for the more common and the false path for the less common case for documentation purposes, but in that case I would expected some code comments as well. > What kind of comment are you thinking about? A "this is not a likely > path" type of comment before each mention of the flag is usually not > very useful. Indeed. It's also not true here. If the workload uses dmabufs, the path obviously is very likely.