In any case, instead of changing st_dev and st_ino or changing the world to
work with file handles, why not add inode generation (and maybe subvol id) to
statx().
filesystem that care enough will provide this information and tools that care
enough will use it.

And how is NFS (especially V2 and V3 - V4.2 at least can add attributes) going to provide these values for statx if applications are going to start depending on them, and especially, will this work for those applications that need to distinguish inodes that are working on an NFS exported btrfs filesystem?

Frank