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

[Vivek Goyal]

On Thu, Apr 30, 2020 at 03:20:13PM +0100, Dr. David Alan Gilbert wrote:
> * 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

Aha.. we are on same page now.

So we need to figure out what's needed to propagate mounts inside guest.
And that's what Max is working on, I think. 

Thanks
Vivek