Re: [PATCH] virtiofsd: Show submounts

[Vivek Goyal]

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?

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.

Vivek