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=-7.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 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 3ACE4C4332E for ; Wed, 23 Dec 2020 20:25:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1FD54223E4 for ; Wed, 23 Dec 2020 20:25:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728395AbgLWUYp (ORCPT ); Wed, 23 Dec 2020 15:24:45 -0500 Received: from mail-1.ca.inter.net ([208.85.220.69]:57480 "EHLO mail-1.ca.inter.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727195AbgLWUYo (ORCPT ); Wed, 23 Dec 2020 15:24:44 -0500 Received: from localhost (offload-3.ca.inter.net [208.85.220.70]) by mail-1.ca.inter.net (Postfix) with ESMTP id CCEBC2EA28A; Wed, 23 Dec 2020 15:24:02 -0500 (EST) Received: from mail-1.ca.inter.net ([208.85.220.69]) by localhost (offload-3.ca.inter.net [208.85.220.70]) (amavisd-new, port 10024) with ESMTP id 0NtYbbCfRz7B; Wed, 23 Dec 2020 15:12:11 -0500 (EST) Received: from [192.168.48.23] (host-104-157-204-209.dyn.295.ca [104.157.204.209]) (using TLSv1 with cipher DHE-RSA-AES128-SHA (128/128 bits)) (No client certificate requested) (Authenticated sender: dgilbert@interlog.com) by mail-1.ca.inter.net (Postfix) with ESMTPSA id 169332EA054; Wed, 23 Dec 2020 15:23:58 -0500 (EST) Reply-To: dgilbert@interlog.com Subject: Re: [PATCH v1 0/6] no-copy bvec To: James Bottomley , Christoph Hellwig , Pavel Begunkov Cc: Ming Lei , linux-block@vger.kernel.org, Jens Axboe , Matthew Wilcox , Johannes Weiner , Alexander Viro , "Darrick J . Wong" , "Martin K . Petersen" , Jonathan Corbet , linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, io-uring@vger.kernel.org, linux-kernel@vger.kernel.org, target-devel@vger.kernel.org, linux-scsi@vger.kernel.org, linux-doc@vger.kernel.org References: <20201215014114.GA1777020@T590> <103235c1-e7d0-0b55-65d0-013d1a09304e@gmail.com> <20201215120357.GA1798021@T590> <20201222141112.GE13079@infradead.org> <933030f0-e428-18fd-4668-68db4f14b976@gmail.com> <20201223155145.GA5902@infradead.org> From: Douglas Gilbert Message-ID: <8abc56c2-4db8-5ee3-ab2d-8960d0eeeb0d@interlog.com> Date: Wed, 23 Dec 2020 15:23:57 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-CA Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 2020-12-23 11:04 a.m., James Bottomley wrote: > On Wed, 2020-12-23 at 15:51 +0000, Christoph Hellwig wrote: >> On Wed, Dec 23, 2020 at 12:52:59PM +0000, Pavel Begunkov wrote: >>> Can scatterlist have 0-len entries? Those are directly translated >>> into bvecs, e.g. in nvme/target/io-cmd-file.c and >>> target/target_core_file.c. I've audited most of others by this >>> moment, they're fine. >> >> For block layer SGLs we should never see them, and for nvme neither. >> I think the same is true for the SCSI target code, but please double >> check. > > Right, no-one ever wants to see a 0-len scatter list entry. The reason > is that every driver uses the sgl to program the device DMA engine in > the way NVME does. a 0 length sgl would be a dangerous corner case: > some DMA engines would ignore it and others would go haywire, so if we > ever let a 0 length list down into the driver, they'd have to > understand the corner case behaviour of their DMA engine and filter it > accordingly, which is why we disallow them in the upper levels, since > they're effective nops anyway. When using scatter gather lists at the far end (i.e. on the storage device) the T10 examples (WRITE SCATTERED and POPULATE TOKEN in SBC-4) explicitly allow the "number of logical blocks" in their sgl_s to be zero and state that it is _not_ to be considered an error. Doug Gilbert