It would be useful to allow reverse pathname lookup of OST FIDs via "lfs fid2path /mntpt <fid>", since the OST FIDs are sometimes referenced in console error messages, and it is useful to be able to do a pathname lookup without first having to manually run e.g. "debugfs -c -R 'stat <O/<f_seq>/d$((<f_oid> %32))/<f_oid>' /dev/<ostdev>" or "ll_decode_filter_fid" on the underlying OST.

The client would need to first determine that this is an OST FID from the FLDB, then fetch the "fid" xattr that contains the MDT parent FID from the OST object (if it exists), and then do the normal fid2path lookup on the returned MDT parent FID.