From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752152AbbAMXak (ORCPT ); Tue, 13 Jan 2015 18:30:40 -0500 Received: from mx0a-00082601.pphosted.com ([67.231.145.42]:17246 "EHLO mx0a-00082601.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751145AbbAMXai (ORCPT ); Tue, 13 Jan 2015 18:30:38 -0500 Message-ID: <54B5AA84.9010306@fb.com> Date: Tue, 13 Jan 2015 16:30:12 -0700 From: Jens Axboe User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.3.0 MIME-Version: 1.0 To: Richard Weinberger , Christoph Hellwig CC: , , , , , Subject: Re: [PATCH 2/2 v2] UBI: Block: Add blk-mq support References: <1420926734-16417-1-git-send-email-richard@nod.at> <1420926734-16417-2-git-send-email-richard@nod.at> <20150113162553.GA24351@infradead.org> <54B5A188.2000001@nod.at> <54B5A213.1090502@fb.com> <54B5A77C.9040405@nod.at> In-Reply-To: <54B5A77C.9040405@nod.at> Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: 7bit X-Originating-IP: [192.168.57.29] X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:5.13.68,1.0.33,0.0.0000 definitions=2015-01-13_07:2015-01-13,2015-01-13,1970-01-01 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 spamscore=0 suspectscore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=7.0.1-1402240000 definitions=main-1501130230 X-FB-Internal: deliver Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/13/2015 04:17 PM, Richard Weinberger wrote: > Am 13.01.2015 um 23:54 schrieb Jens Axboe: >>>> blk_rq_map_sg returns the number of entries actually mapped, which >>>> might be smaller than the number passed in due to merging. >>> >>> Yep, but the ubi_sql has a fixed number of scatterlist entries, UBI_MAX_SG_COUNT. >>> And I limit it also to that using: blk_queue_max_segments(dev->rq, UBI_MAX_SG_COUNT); >>> >>> Is there another reason why I should use the return value of blk_rq_map_sg()? >>> Please also note that the UBI block driver is read-only. >> >> It can return less than what you asked for, if segments are coalesced. >> Read/write, doesn't matter. You should always use the returned value as >> the indication of how many segments to access in pdu->usgl.sg for data >> transfer. > > Sorry, I don't fully understand. > > Currently the driver does: > to_read = blk_rq_bytes(req); > Then it fills pdu->usgl.sg up to to_read bytes > and calls blk_mq_end_request(). > > If I understand you correctly it can happen that blk_rq_bytes() returns > more bytes than blk_rq_map_sg() allocated, right? No, the number of bytes will be the same, no magic is involved :-) But lets say the initial request has 4 bios, with each 2 pages, for a total of 8 segments. Lets further assume that the pages in each bio are contiguous, so that blk_rq_map_sg() will map this to 4 sg elements, each 2xpages long. Now, this may already be handled just fine, and you don't need to update/store the actual sg count. I just looked at the source, and I'm assuming it'll do the right thing (ubi_read_sg() will bump the active sg element, when that size has been consumed), but I don't have ubi_read_sg() in my tree to verify. -- Jens Axboe