How to support directory opacity in a filesystem for overlayfs to use?

[David Howells]

Hi Miklós,

What's the best way for a network filesystem to make a native
directory-is-opaque flag available to the system?  Is it best to catch
setxattr/getxattr/removexattr("overlay.opaque") and translate these into the
RPCs to wrangle the flag?

Thanks,
David