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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=unavailable 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 34B0CC43610 for ; Thu, 22 Nov 2018 01:17:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id F00BF20645 for ; Thu, 22 Nov 2018 01:17:32 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org F00BF20645 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2391322AbeKVLy0 (ORCPT ); Thu, 22 Nov 2018 06:54:26 -0500 Received: from mx1.redhat.com ([209.132.183.28]:38064 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732495AbeKVLy0 (ORCPT ); Thu, 22 Nov 2018 06:54:26 -0500 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id BEAE7792B7; Thu, 22 Nov 2018 01:17:30 +0000 (UTC) Received: from ming.t460p (ovpn-8-27.pek2.redhat.com [10.72.8.27]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 8687A600D6; Thu, 22 Nov 2018 01:17:10 +0000 (UTC) Date: Thu, 22 Nov 2018 09:17:06 +0800 From: Ming Lei To: Christoph Hellwig Cc: Jens Axboe , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Theodore Ts'o , Omar Sandoval , Sagi Grimberg , Dave Chinner , Kent Overstreet , Mike Snitzer , dm-devel@redhat.com, Alexander Viro , linux-fsdevel@vger.kernel.org, Shaohua Li , linux-raid@vger.kernel.org, David Sterba , linux-btrfs@vger.kernel.org, "Darrick J . Wong" , linux-xfs@vger.kernel.org, Gao Xiang , linux-ext4@vger.kernel.org, Coly Li , linux-bcache@vger.kernel.org, Boaz Harrosh , Bob Peterson , cluster-devel@redhat.com Subject: Re: [PATCH V11 03/19] block: introduce bio_for_each_bvec() Message-ID: <20181122011705.GC20814@ming.t460p> References: <20181121032327.8434-1-ming.lei@redhat.com> <20181121032327.8434-4-ming.lei@redhat.com> <20181121133244.GB1640@lst.de> <20181121153135.GB19111@ming.t460p> <20181121161025.GB4977@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181121161025.GB4977@lst.de> User-Agent: Mutt/1.9.1 (2017-09-22) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.39]); Thu, 22 Nov 2018 01:17:31 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 21, 2018 at 05:10:25PM +0100, Christoph Hellwig wrote: > On Wed, Nov 21, 2018 at 11:31:36PM +0800, Ming Lei wrote: > > > But while looking over this I wonder why we even need the max_seg_len > > > here. The only thing __bvec_iter_advance does it to move bi_bvec_done > > > and bi_idx forward, with corresponding decrements of bi_size. As far > > > as I can tell the only thing that max_seg_len does is that we need > > > to more iterations of the while loop to archive the same thing. > > > > > > And actual bvec used by the caller will be obtained using > > > bvec_iter_bvec or segment_iter_bvec depending on if they want multi-page > > > or single-page variants. > > > > Right, we let __bvec_iter_advance() serve for both multi-page and single-page > > case, then we have to tell it via one way or another, now we use the constant > > of 'max_seg_len'. > > > > Or you suggest to implement two versions of __bvec_iter_advance()? > > No - I think we can always use the code without any segment in > bvec_iter_advance. Because bvec_iter_advance only operates on the > iteractor, the generation of an actual single-page or multi-page > bvec is left to the caller using the bvec_iter_bvec or segment_iter_bvec > helpers. The only difference is how many bytes you can move the > iterator forward in a single loop iteration - so if you pass in > PAGE_SIZE as the max_seg_len you just will have to loop more often > for a large enough bytes, but not actually do anything different. Yeah, I see that. The difference is made by bio_iter_iovec()/bio_iter_mp_iovec() in __bio_for_each_segment()/__bio_for_each_bvec(). Thanks, Ming