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 X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id AF7E2C4361B for ; Wed, 9 Dec 2020 17:07:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 78D8023C44 for ; Wed, 9 Dec 2020 17:07:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731993AbgLIRHK (ORCPT ); Wed, 9 Dec 2020 12:07:10 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60824 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731151AbgLIRHK (ORCPT ); Wed, 9 Dec 2020 12:07:10 -0500 Received: from ZenIV.linux.org.uk (zeniv.linux.org.uk [IPv6:2002:c35c:fd02::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D8DD2C0613CF; Wed, 9 Dec 2020 09:06:29 -0800 (PST) Received: from viro by ZenIV.linux.org.uk with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1kn2v4-0006Lh-BZ; Wed, 09 Dec 2020 17:06:26 +0000 Date: Wed, 9 Dec 2020 17:06:26 +0000 From: Al Viro To: Pavel Begunkov Cc: Jens Axboe , linux-fsdevel@vger.kernel.org, io-uring@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, Christoph Hellwig , Matthew Wilcox , Ming Lei , Johannes Weiner Subject: Re: [RFC 0/2] nocopy bvec for direct IO Message-ID: <20201209170626.GO3579531@ZenIV.linux.org.uk> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: Al Viro Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Wed, Dec 09, 2020 at 02:19:50AM +0000, Pavel Begunkov wrote: > The idea is to avoid copying, merging, etc. bvec from iterator to bio > in direct I/O and use the one we've already got. Hook it up for io_uring. > Had an eye on it for a long, and it also was brought up by Matthew > just recently. Let me know if I forgot or misplaced some tags. > > A benchmark got me 430KIOPS vs 540KIOPS, or +25% on bare metal. And perf > shows that bio_iov_iter_get_pages() was taking ~20%. The test is pretty > silly, but still imposing. I'll redo it closer to reality for next > iteration, anyway need to double check some cases. > > If same applied to iomap, common chunck can be moved from block_dev > into bio_iov_iter_get_pages(), but if there any benefit for filesystems, > they should explicitly opt in with ITER_BVEC_FLAG_FIXED. To reiterate what hch said - this "opt in" is wrong. Out-of-tree code that does async IO on bvec-backed iov_iter, setting it up on its own will have to adapt, that all. iov_iter and its users are already in serious need of simplification and cleanups; piling more on top of that would be a bloody bad idea. Proposed semantics change for bvec-backed iov_iter makes a lot of sense, so let's make sure that everything in tree can live with it, document the change and switch to better semantics. This thing should be unconditional. Document it in D/f/porting and if something out of tree complains, it's their problem - not ours.