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=-8.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_MUTT autolearn=ham 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 85189C04AAF for ; Tue, 21 May 2019 08:05:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5F1D221773 for ; Tue, 21 May 2019 08:05:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726204AbfEUIFa (ORCPT ); Tue, 21 May 2019 04:05:30 -0400 Received: from mx1.redhat.com ([209.132.183.28]:33002 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726138AbfEUIFa (ORCPT ); Tue, 21 May 2019 04:05:30 -0400 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 71BB330BB546; Tue, 21 May 2019 08:05:30 +0000 (UTC) Received: from ming.t460p (ovpn-8-24.pek2.redhat.com [10.72.8.24]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 955A06085B; Tue, 21 May 2019 08:05:25 +0000 (UTC) Date: Tue, 21 May 2019 16:05:20 +0800 From: Ming Lei To: Christoph Hellwig Cc: axboe@fb.com, linux-block@vger.kernel.org, Hannes Reinecke Subject: Re: [PATCH 1/4] block: don't decrement nr_phys_segments for physically contigous segments Message-ID: <20190521080519.GB27095@ming.t460p> References: <20190521070143.22631-1-hch@lst.de> <20190521070143.22631-2-hch@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190521070143.22631-2-hch@lst.de> User-Agent: Mutt/1.11.3 (2019-02-01) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.49]); Tue, 21 May 2019 08:05:30 +0000 (UTC) Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Tue, May 21, 2019 at 09:01:40AM +0200, Christoph Hellwig wrote: > Currently ll_merge_requests_fn, unlike all other merge functions, > reduces nr_phys_segments by one if the last segment of the previous, > and the first segment of the next segement are contigous. While this > seems like a nice solution to avoid building smaller than possible > requests it causes a mismatch between the segments actually present > in the request and those iterated over by the bvec iterators, including > __rq_for_each_bio. This can for example mistrigger the single segment > optimization in the nvme-pci driver, and might lead to mismatching > nr_phys_segments number when recalculating the number of request > when inserting a cloned request. > > We could possibly work around this by making the bvec iterators take > the front and back segment size into account, but that would require > moving them from the bio to the bio_iter and spreading this mess > over all users of bvecs. Or we could simply remove this optimization > under the assumption that most users already build good enough bvecs, > and that the bio merge patch never cared about this optimization > either. The latter is what this patch does. > > dff824b2aadb ("nvme-pci: optimize mapping of small single segment requests"). > Signed-off-by: Christoph Hellwig > Reviewed-by: Hannes Reinecke > --- > block/blk-merge.c | 23 +---------------------- > 1 file changed, 1 insertion(+), 22 deletions(-) > > diff --git a/block/blk-merge.c b/block/blk-merge.c > index 21e87a714a73..80a5a0facb87 100644 > --- a/block/blk-merge.c > +++ b/block/blk-merge.c > @@ -358,7 +358,6 @@ static unsigned int __blk_recalc_rq_segments(struct request_queue *q, > unsigned front_seg_size; > struct bio *fbio, *bbio; > struct bvec_iter iter; > - bool new_bio = false; > > if (!bio) > return 0; > @@ -379,31 +378,12 @@ static unsigned int __blk_recalc_rq_segments(struct request_queue *q, > nr_phys_segs = 0; > for_each_bio(bio) { > bio_for_each_bvec(bv, bio, iter) { > - if (new_bio) { > - if (seg_size + bv.bv_len > - > queue_max_segment_size(q)) > - goto new_segment; > - if (!biovec_phys_mergeable(q, &bvprv, &bv)) > - goto new_segment; > - > - seg_size += bv.bv_len; > - > - if (nr_phys_segs == 1 && seg_size > > - front_seg_size) > - front_seg_size = seg_size; > - > - continue; > - } > -new_segment: > bvec_split_segs(q, &bv, &nr_phys_segs, &seg_size, > &front_seg_size, NULL, UINT_MAX); > - new_bio = false; > } > bbio = bio; > - if (likely(bio->bi_iter.bi_size)) { > + if (likely(bio->bi_iter.bi_size)) > bvprv = bv; > - new_bio = true; > - } > } > > fbio->bi_seg_front_size = front_seg_size; > @@ -725,7 +705,6 @@ static int ll_merge_requests_fn(struct request_queue *q, struct request *req, > req->bio->bi_seg_front_size = seg_size; > if (next->nr_phys_segments == 1) > next->biotail->bi_seg_back_size = seg_size; > - total_phys_segments--; > } > > if (total_phys_segments > queue_max_segments(q)) > -- > 2.20.1 > Reviewed-by: Ming Lei Thanks, Ming