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=-3.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED 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 59B09C433ED for ; Fri, 21 May 2021 10:06:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3B92D6135C for ; Fri, 21 May 2021 10:06:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233826AbhEUKIL convert rfc822-to-8bit (ORCPT ); Fri, 21 May 2021 06:08:11 -0400 Received: from us-smtp-delivery-44.mimecast.com ([207.211.30.44]:51079 "EHLO us-smtp-delivery-44.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233670AbhEUKIC (ORCPT ); Fri, 21 May 2021 06:08:02 -0400 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-321-nzPTwh8TNoKqhAUNAmkdWg-1; Fri, 21 May 2021 06:06:33 -0400 X-MC-Unique: nzPTwh8TNoKqhAUNAmkdWg-1 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 mimecast-mx01.redhat.com (Postfix) with ESMTPS id 703E56D241; Fri, 21 May 2021 10:06:32 +0000 (UTC) Received: from bahia.lan (ovpn-112-49.ams2.redhat.com [10.36.112.49]) by smtp.corp.redhat.com (Postfix) with ESMTP id CFCBB1964B; Fri, 21 May 2021 10:06:17 +0000 (UTC) Date: Fri, 21 May 2021 12:06:16 +0200 From: Greg Kurz To: Miklos Szeredi Cc: virtualization@lists.linux-foundation.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, virtio-fs-list , Stefan Hajnoczi , Max Reitz , Vivek Goyal Subject: Re: [PATCH v4 4/5] virtiofs: Skip submounts in sget_fc() Message-ID: <20210521120616.49d52565@bahia.lan> In-Reply-To: References: <20210520154654.1791183-1-groug@kaod.org> <20210520154654.1791183-5-groug@kaod.org> <20210521103921.153a243d@bahia.lan> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: kaod.org Content-Type: text/plain; charset=WINDOWS-1252 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Fri, 21 May 2021 10:50:34 +0200 Miklos Szeredi wrote: > On Fri, 21 May 2021 at 10:39, Greg Kurz wrote: > > > > On Fri, 21 May 2021 10:26:27 +0200 > > Miklos Szeredi wrote: > > > > > On Thu, 20 May 2021 at 17:47, Greg Kurz wrote: > > > > > > > > All submounts share the same virtio-fs device instance as the root > > > > mount. If the same virtiofs filesystem is mounted again, sget_fc() > > > > is likely to pick up any of these submounts and reuse it instead of > > > > the root mount. > > > > > > > > On the server side: > > > > > > > > # mkdir ${some_dir} > > > > # mkdir ${some_dir}/mnt1 > > > > # mount -t tmpfs none ${some_dir}/mnt1 > > > > # touch ${some_dir}/mnt1/THIS_IS_MNT1 > > > > # mkdir ${some_dir}/mnt2 > > > > # mount -t tmpfs none ${some_dir}/mnt2 > > > > # touch ${some_dir}/mnt2/THIS_IS_MNT2 > > > > > > > > On the client side: > > > > > > > > # mkdir /mnt/virtiofs1 > > > > # mount -t virtiofs myfs /mnt/virtiofs1 > > > > # ls /mnt/virtiofs1 > > > > mnt1 mnt2 > > > > # grep virtiofs /proc/mounts > > > > myfs /mnt/virtiofs1 virtiofs rw,seclabel,relatime 0 0 > > > > none on /mnt/mnt1 type virtiofs (rw,relatime,seclabel) > > > > none on /mnt/mnt2 type virtiofs (rw,relatime,seclabel) > > > > > > > > And now remount it again: > > > > > > > > # mount -t virtiofs myfs /mnt/virtiofs2 > > > > # grep virtiofs /proc/mounts > > > > myfs /mnt/virtiofs1 virtiofs rw,seclabel,relatime 0 0 > > > > none on /mnt/mnt1 type virtiofs (rw,relatime,seclabel) > > > > none on /mnt/mnt2 type virtiofs (rw,relatime,seclabel) > > > > myfs /mnt/virtiofs2 virtiofs rw,seclabel,relatime 0 0 > > > > # ls /mnt/virtiofs2 > > > > THIS_IS_MNT2 > > > > > > > > Submount mnt2 was picked-up instead of the root mount. > > > > > > > > Why is this a problem? > > > > > > > It seems very weird to mount the same filesystem again > > and to end up in one of its submounts. We should have: > > > > # ls /mnt/virtiofs2 > > mnt1 mnt2 > > Okay, sorry, I understand the problem. The solution is wrong, > however: the position of the submount on that list is no indication > that it's the right one (it's possible that the root sb will go away > and only a sub-sb will remain). > Ah... I had myself convinced this could not happen, i.e. you can't unmount a parent sb with a sub-sb still mounted. How can this happen ? > Even just setting a flag in the root, indicating that it's the root > isn't fully going to solve the problem. > > Here's issue in full: > > case 1: no connection for "myfs" exists > - need to create fuse_conn, sb > > case 2: connection for "myfs" exists but only sb for submount How would we know this sb isn't a root sb ? > - only create sb for root, reuse fuse_conn > > case 3: connection for "myfs" as well as root sb exists > - reuse sb > > I'll think about how to fix this properly, it's probably going to be > rather more involved... > Sure. BTW I'm wondering why we never reuse sbs for submounts ? > Thanks, > Miklos