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Thu, 30 Apr 2020 14:47:14 +0000 (UTC) Received: from redhat.com (unknown [10.36.110.44]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 80FD02B4A5; Thu, 30 Apr 2020 14:47:07 +0000 (UTC) Date: Thu, 30 Apr 2020 15:47:04 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Vivek Goyal Subject: Re: [PATCH] virtiofsd: Show submounts Message-ID: <20200430144704.GG2184629@redhat.com> References: <20200424133516.73077-1-mreitz@redhat.com> <20200427175902.GM2923@work-vm> <20200429145720.GA2835@work-vm> <8c73f374-fcc8-1684-b581-84a9ab501aa9@redhat.com> <20200430085812.GC2874@work-vm> <20200430135639.GA260081@redhat.com> <20200430142013.GI2874@work-vm> <20200430143425.GD2184629@redhat.com> <20200430144116.GD260081@redhat.com> MIME-Version: 1.0 In-Reply-To: <20200430144116.GD260081@redhat.com> User-Agent: Mutt/1.13.3 (2020-01-12) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline Received-SPF: pass client-ip=205.139.110.61; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/04/30 01:04:40 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] X-Received-From: 205.139.110.61 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: virtio-fs@redhat.com, Max Reitz , "Dr. David Alan Gilbert" , Stefan Hajnoczi , qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, Apr 30, 2020 at 10:41:16AM -0400, Vivek Goyal wrote: > On Thu, Apr 30, 2020 at 03:34:25PM +0100, Daniel P. Berrang=C3=A9 wrote: > > On Thu, Apr 30, 2020 at 03:20:13PM +0100, Dr. David Alan Gilbert wrote: > > > * Vivek Goyal (vgoyal@redhat.com) wrote: > > > > On Thu, Apr 30, 2020 at 09:58:12AM +0100, Dr. David Alan Gilbert wr= ote: > > > > [..] > > > > > > > Even without this patch, the SLAVE stuff worked so if you sta= rt the > > > > > > > daemon and *then* mount under the shared directory, the guest= sees it > > > > > > > with or without this patch. > > > > > >=20 > > > > > > Hm, I don=E2=80=99t. Do you really? > > > > >=20 > > > > > Yes! With your patch reverted: > > > > >=20 > > > > > Start virtiofsd, mount in the guest: > > > > >=20 > > > > > host: > > > > > # ./virtiofsd --socket-path=3D/tmp/vhostqemu -o source=3D/home/dg= ilbert/virtio-fs/fs -o log_level=3Dwarn -o no_writeback > > > > >=20 > > > > > guest: > > > > > # mount -t virtiofs myfs /sysroot > > > > >=20 > > > > > host: > > > > > # findmnt -o +PROPAGATION -N 6100 > > > > > TARGET SOURCE = FSTYPE OPTIONS = PROPAGATION > > > > > / /dev/mapper/fedora_dgilbert--t580-root[/home/dgilbert/virt= io-fs/fs] xfs rw,relatime,seclabel,attr2,inode64,logbufs=3D8,logbsize=3D= 32k,no private,slave > > > > > # mount -t tmpfs /dev/null /home/dgilbert/virtio-fs/fs/tmp > > > > > # findmnt -o +PROPAGATION -N 6100 > > > > > TARGET SOURCE = FSTYPE OPTIONS = PROPAGATION > > > > > / /dev/mapper/fedora_dgilbert--t580-root[/home/dgilbert/virt= io-fs/fs] xfs rw,relatime,seclabel,attr2,inode64,logbufs=3D8,logbsize=3D= 32k,no private,slave > > > > > =E2=94=94=E2=94=80/tmp /dev/null = tmpfs rw,relatime,seclabel = private,slave > > > >=20 > > > > Why is it showing a mount point at "/tmp". If mount point propagate= d, then > > > > inside guest we should see a mount point at /sysroot/tmp? > > >=20 > > > That findmnt is on the host. > > >=20 > > > > So there are two things. > > > >=20 > > > > A. Propagation of mount from host to virtiofsd. > > > > B. Visibility of that mount inside guest over fuse protocol (submou= nt > > > > functionality). > > > >=20 > > > > I think A works for me without any patches. But don't think B is wo= rking > > > > for me. I don't see the submount inside guest.=20 > > > >=20 > > > > > # touch /home/dgilbert/virtio-fs/fs/tmp/hello > > > > >=20 > > > > > guest: > > > > > # ls -l /sysroot/tmp > > > > > total 0 > > > > > -rw-r--r-- 1 root root 0 Apr 30 08:50 hello > > > >=20 > > > > Do a "findmnt /sysroot/tmp" inside guest and see what do you see. > > > >=20 > > > > You will be able to see "hello" as long as virtiofsd sees the new > > > > mount point, I think. And guest does not have to see that mount poi= nt > > > > for this simple test to work. > > >=20 > > > Right, the guest just sees: > > >=20 > > > `-/sysroot myfs virtiof rw,relatime > >=20 > > That is a good thing surely ? If I'm exporting "/sysroot" from the host= , > > I want the content in "/sysroot/some/sub/mount" to be visible to the > > guest, but I don't want the guest to see "/sysroot/some/sub/mount" > > as an actual mount point. That would be leaking information about the > > host storage setup into the guest. The host admin should be free to > > re-arrange submounts in the host OS, to bring more storage space online= , > > and have this be transparent to the guest OS. >=20 > If we don't see mount inside guest, we run into the possibility of inode > number collision. On host two files in shared dir can have same inode > number (if they are on two different filesystem with different device > numbers). But inside guest, we will show device number of virtiofs, > and it will look as if two files in this filesystem have same inode > number, breaking some workloads. >=20 > By propagating mounts (submounts), we can assign a unique device number > to these submounts and hence number pair will become unique. Ah, yes, that's true. In 9pfs there was recent changes precisely because of this clash possibility: commit 1a6ed33cc56997479bbe5b48337ff8da44585bd4 Author: Antonios Motakis Date: Thu Oct 10 11:36:05 2019 +0200 9p: Added virtfs option 'multidevs=3Dremap|forbid|warn' =20 'warn' (default): Only log an error message (once) on host if more than= one device is shared by same export, except of that just ignore this config error though. This is the default behaviour for not breaking existing installations implying that they really know what they are doing. =20 'forbid': Like 'warn', but except of just logging an error this also denies access of guest to additional devices. =20 'remap': Allows to share more than one device per export by remapping inodes from host to guest appropriately. To support multiple devices on= the 9p share, and avoid qid path collisions we take the device id as input = to generate a unique QID path. The lowest 48 bits of the path will be set equal to the file inode, and the top bits will be uniquely assigned bas= ed on the top 16 bits of the inode and the device id. Perhaps we should try to support the same options in virtio-fs. At least the "forbid" and "remap" options make sense I think. "warn" was only really there for backcompat. If we can expose it to the guest, then a further "expose" option would be viable. Regards, Daniel --=20 |: https://berrange.com -o- https://www.flickr.com/photos/dberrange= :| |: https://libvirt.org -o- https://fstop138.berrange.com= :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange= :|