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

[Vivek Goyal]

On Wed, May 06, 2020 at 05:32:58PM +0200, Max Reitz 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.
> 
> 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.

Hi Max,

What's duplicate mount. Is it two bind bounts created from same
filesystem.

So two bind mounts which have same st_dev on host, will appear to come
from two different devices in guest (different st_dev), right?

Vivek

> 
> 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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