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=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 7B9E7C04AA7 for ; Mon, 13 May 2019 12:04:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 45AEA20989 for ; Mon, 13 May 2019 12:04:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727861AbfEMMEF (ORCPT ); Mon, 13 May 2019 08:04:05 -0400 Received: from verein.lst.de ([213.95.11.211]:38860 "EHLO newverein.lst.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727487AbfEMMEF (ORCPT ); Mon, 13 May 2019 08:04:05 -0400 Received: by newverein.lst.de (Postfix, from userid 2407) id 45B6B68AFE; Mon, 13 May 2019 14:03:45 +0200 (CEST) Date: Mon, 13 May 2019 14:03:44 +0200 From: Christoph Hellwig To: Ming Lei Cc: Christoph Hellwig , axboe@fb.com, Matias Bjorling , linux-block@vger.kernel.org Subject: Re: [PATCH 01/10] block: don't decrement nr_phys_segments for physically contigous segments Message-ID: <20190513120344.GA22993@lst.de> References: <20190513063754.1520-1-hch@lst.de> <20190513063754.1520-2-hch@lst.de> <20190513094544.GA30381@ming.t460p> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190513094544.GA30381@ming.t460p> User-Agent: Mutt/1.5.17 (2007-11-01) Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Mon, May 13, 2019 at 05:45:45PM +0800, Ming Lei wrote: > On Mon, May 13, 2019 at 08:37:45AM +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 > > Some workloads need this optimization, please see 729204ef49ec00b > ("block: relax check on sg gap"): And we still allow to merge the segments with this patch. The only difference is that these merges do get accounted as extra segments. > > 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 could cause overwrites of too small kmalloc > > Request based drivers usually shouldn't iterate bio any more. We do that in a couple of places. For one the nvme single segment optimization that triggered this bug. Also for range discard support in nvme and virtio. Then we have loop that iterate the segments, but doesn't use the nr_phys_segments count, and plenty of others that iterate over pages at the moment but should be iterating bvecs, e.g. ubd or aoe.