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=-2.4 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 D4AE5C2D0DB for ; Thu, 23 Jan 2020 17:28:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A9E3C21734 for ; Thu, 23 Jan 2020 17:28:30 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="hk+zrhQ8" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727022AbgAWR2a (ORCPT ); Thu, 23 Jan 2020 12:28:30 -0500 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:30617 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727590AbgAWR2a (ORCPT ); Thu, 23 Jan 2020 12:28:30 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1579800508; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=uOo86XYx2XlckK7VO6n+fr3sTCswBT7TBWfyx1cJOOc=; b=hk+zrhQ8r8Fy2HN1nhx6DnJ8o+yoXQRn1tAkzCUh/FGH+OIq4VZMfsB0WQVylSzgcfw2gY loS1PAT+UsXyDE04RYosBZ10kRAwQUrmD5lg2Qjn9Vh71lkre9hzXJ+p7ecEZTKT3XiyFA BtCvlZ6zYswFzYAJhTLk0tJdZ42ztbo= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-276-e2SK6Eb_PMGXILK14ERXWA-1; Thu, 23 Jan 2020 12:28:21 -0500 X-MC-Unique: e2SK6Eb_PMGXILK14ERXWA-1 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 mimecast-mx01.redhat.com (Postfix) with ESMTPS id D46061137858; Thu, 23 Jan 2020 17:28:19 +0000 (UTC) Received: from localhost (unknown [10.18.25.174]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 2C8058CCEC; Thu, 23 Jan 2020 17:28:17 +0000 (UTC) Date: Thu, 23 Jan 2020 12:28:16 -0500 From: Mike Snitzer To: Stefan Bader Cc: Jens Axboe , linux-kernel@vger.kernel.org, linux-block@vger.kernel.org, dm-devel@redhat.com, Tyler Hicks , Alasdair Kergon Subject: Re: [PATCH 1/1] blk/core: Gracefully handle unset make_request_fn Message-ID: <20200123172816.GA31063@redhat.com> References: <20200123091713.12623-1-stefan.bader@canonical.com> <20200123091713.12623-2-stefan.bader@canonical.com> <20200123103541.GA28102@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200123103541.GA28102@redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Thu, Jan 23 2020 at 5:35am -0500, Mike Snitzer wrote: > On Thu, Jan 23 2020 at 4:17am -0500, > Stefan Bader wrote: > > > When device-mapper adapted for multi-queue functionality, they > > also re-organized the way the make-request function was set. > > Before, this happened when the device-mapper logical device was > > created. Now it is done once the mapping table gets loaded the > > first time (this also decides whether the block device is request > > or bio based). > > > > However in generic_make_request(), the request function gets used > > without further checks and this happens if one tries to mount such > > a partially set up device. > > > > This can easily be reproduced with the following steps: > > - dmsetup create -n test > > - mount /dev/dm-<#> /mnt > > > > This maybe is something which also should be fixed up in device- > > mapper. > > I'll look closer at other options. > > > But given there is already a check for an unset queue > > pointer and potentially there could be other drivers which do or > > might do the same, it sounds like a good move to add another check > > to generic_make_request_checks() and to bail out if the request > > function has not been set, yet. > > > > BugLink: https://bugs.launchpad.net/bugs/1860231 > > >From that bug; > "The currently proposed fix introduces no chance of stability > regressions. There is a chance of a very small performance regression > since an additional pointer comparison is performed on each block layer > request but this is unlikely to be noticeable." > > This captures my immediate concern: slowing down everyone for this DM > edge-case isn't desirable. SO I had a look and there isn't anything easier than adding the proposed NULL check in generic_make_request_checks(). Given the many conditionals in that function.. what's one more? ;) I looked at marking the queue frozen to prevent IO via blk_queue_enter()'s existing cheeck -- but that quickly felt like an abuse, especially in that there isn't a queue unfreeze for bio-based. Jens, I'll defer to you to judge this patch further. If you're OK with it: cool. If not, I'm open to suggestions for how to proceed.