On Wed, Sep 30, 2015 at 01:26:51PM -0400, Anna Schumaker wrote:

I still want to do an in-kernel copy even if the files are on different
mountpoints, and NFS has a "server to server" copy that expects two
files on different mountpoints.  Let's have individual filesystems
implement this check instead.

NAK.  I thing this is a bad idea in general and will only be convinced
by a properly audited actual implementation.  And even then with a flag
where the file system specificly needs to opt into this behavior instead
of getting it by default.