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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 AA939C433DB for ; Wed, 23 Dec 2020 16:05:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7B31E22AAF for ; Wed, 23 Dec 2020 16:05:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727641AbgLWQFE (ORCPT ); Wed, 23 Dec 2020 11:05:04 -0500 Received: from bedivere.hansenpartnership.com ([96.44.175.130]:34262 "EHLO bedivere.hansenpartnership.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727360AbgLWQFE (ORCPT ); Wed, 23 Dec 2020 11:05:04 -0500 Received: from localhost (localhost [127.0.0.1]) by bedivere.hansenpartnership.com (Postfix) with ESMTP id D32A412809AE; Wed, 23 Dec 2020 08:04:23 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=hansenpartnership.com; s=20151216; t=1608739463; bh=dzRbIAIFFOVOpKnDTOA6zj0KHef4324sFMv5xTBqMv8=; h=Message-ID:Subject:From:To:Date:In-Reply-To:References:From; b=UOJqbOiPCHjby5AESe5JFFEouYk+7tGM0MdXl1UUaO5WknQH6hP4It7J3GKxcEi95 Yte0Ppq4ypfucjFU3ZXlhSB3CzxVj/rcDo+FxKT5WH0PyxzP08G4QrDqX877Em71j9 QH2hzjXQGwyh49OWRTJ42Aqg9mNa9g8Tkf2ap8ls= Received: from bedivere.hansenpartnership.com ([127.0.0.1]) by localhost (bedivere.hansenpartnership.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id c3cuwwSOf488; Wed, 23 Dec 2020 08:04:23 -0800 (PST) Received: from jarvis.int.hansenpartnership.com (c-73-35-198-56.hsd1.wa.comcast.net [73.35.198.56]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by bedivere.hansenpartnership.com (Postfix) with ESMTPSA id D1B9C12809AD; Wed, 23 Dec 2020 08:04:22 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=hansenpartnership.com; s=20151216; t=1608739463; bh=dzRbIAIFFOVOpKnDTOA6zj0KHef4324sFMv5xTBqMv8=; h=Message-ID:Subject:From:To:Date:In-Reply-To:References:From; b=UOJqbOiPCHjby5AESe5JFFEouYk+7tGM0MdXl1UUaO5WknQH6hP4It7J3GKxcEi95 Yte0Ppq4ypfucjFU3ZXlhSB3CzxVj/rcDo+FxKT5WH0PyxzP08G4QrDqX877Em71j9 QH2hzjXQGwyh49OWRTJ42Aqg9mNa9g8Tkf2ap8ls= Message-ID: Subject: Re: [PATCH v1 0/6] no-copy bvec From: James Bottomley To: 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 Date: Wed, 23 Dec 2020 08:04:22 -0800 In-Reply-To: <20201223155145.GA5902@infradead.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> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.34.4 MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org 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. James