[Virtio-fs] [PATCH] virtiofsd: Show submounts

[Dr. David Alan Gilbert]

* Vivek Goyal (vgoyal at redhat.com) wrote:
> On Thu, Apr 30, 2020 at 09:58:12AM +0100, Dr. David Alan Gilbert wrote:
> [..]
> > > > Even without this patch, the SLAVE stuff worked so if you start the
> > > > daemon and *then* mount under the shared directory, the guest sees it
> > > > with or without this patch.
> > > 
> > > Hm, I don’t.  Do you really?
> > 
> > Yes! With your patch reverted:
> > 
> > Start virtiofsd, mount in the guest:
> > 
> > host:
> > # ./virtiofsd --socket-path=/tmp/vhostqemu -o source=/home/dgilbert/virtio-fs/fs  -o log_level=warn -o no_writeback
> > 
> > guest:
> > # mount -t virtiofs myfs /sysroot
> > 
> > host:
> > # findmnt -o +PROPAGATION -N 6100
> > TARGET SOURCE                                                              FSTYPE OPTIONS                                                      PROPAGATION
> > /      /dev/mapper/fedora_dgilbert--t580-root[/home/dgilbert/virtio-fs/fs] xfs    rw,relatime,seclabel,attr2,inode64,logbufs=8,logbsize=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/virtio-fs/fs] xfs    rw,relatime,seclabel,attr2,inode64,logbufs=8,logbsize=32k,no private,slave
> > └─/tmp /dev/null                                                           tmpfs  rw,relatime,seclabel                                         private,slave
> 
> Why is it showing a mount point at "/tmp". If mount point propagated, then
> inside guest we should see a mount point at /sysroot/tmp?

That findmnt is on the host.

> So there are two things.
> 
> A. Propagation of mount from host to virtiofsd.
> B. Visibility of that mount inside guest over fuse protocol (submount
>   functionality).
> 
> I think A works for me without any patches. But don't think B is working
> for me. I don't see the submount inside guest. 
> 
> > # touch /home/dgilbert/virtio-fs/fs/tmp/hello
> > 
> > guest:
> > # ls -l /sysroot/tmp
> > total 0
> > -rw-r--r-- 1 root root 0 Apr 30 08:50 hello
> 
> Do a "findmnt /sysroot/tmp" inside guest and see what do you see.
> 
> 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 point
> for this simple test to work.

Right, the guest just sees:

`-/sysroot                            myfs       virtiof rw,relatime

Dave

> Vivek
--
Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK