NFS and FUSE report a zero f_fsid, so another export or another FUSE mount at the
same point was named alike and kept without a warrant; xfs's f_fsid follows the
device number. A filesystem is now named by its UUID (its device matched in
/dev/disk/by-uuid), else a nonzero f_fsid, else mountinfo's source; one that cannot
be named is never kept or taken. Entries left on a filesystem not back within 30 days
are forgotten, said once, and the naming read is shared per mount.