[Virtio-fs] [RFC] Duplicate submounts in the guest

[Dr. David Alan Gilbert]

* Max Reitz (mreitz at redhat.com) wrote:
> Hi,
> 
> This RFC consists of two patch series, one for virtiofsd (i.e., to be
> applied to the qemu repo), and one for Linux.
> 
> The problem it wants to solve is that virtiofsd just passes through the
> stat.st_ino value from the host to the guest, without taking stat.st_dev
> into account.  Therefore, it is generally impossible to uniquely
> identify nodes in the guest by their st_dev/st_ino combinations.
> 
> One way to fix this would be to derive some algorithm to derive a unique
> st_ino value from the st_dev/st_ino combination on the host, but this is
> not always possible.
> 
> So what this series does it basically passing through st_dev alongside
> st_ino: We can let the guest create auto-mounted submounts for every
> host mount point, so that they get their own dedicated st_dev.  This is
> similar to what NFS does with the crossmnt option.

Nice;  this should probably get cc'd to fuse-devel and/or put as a pull
into their github to see what they have to say.

Dave

> Announcing submounts to the guest (the FUSE client) is done by the FUSE
> server (virtiofsd) of setting st_rdev to a non-zero value (e.g. st_dev)
> for every mount point.
> 
> In this version here, duplicate mounts on the host (same st_dev) will
> not necessarily have the same st_dev in the guest.  That would be more
> complicated to implement, and there is probably no real need to.
> 
> Furthermore, note that virtiofsd identifies files based on
> st_dev/st_ino, so if you do have duplicate mounts on the guest, it is
> very much possible that you will run into problems when trying to pass
> them through to the guest (because virtiofsd may treat them as one and
> the same tree).
> 
> -- 
> 2.26.2
> 
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--
Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK