Ceph » CephFS

I started looking at this today, and it's a little trickier than I thought. The current client code that sends the LOOKUPINO request (_lookup_ino) always just leaves the snapid field zeroed out (AFAICT).

The corresponding handler on the MDS (Server::handle_client_lookup_ino), seems to rely on that though:

  if ((uint64_t)req->head.args.lookupino.snapid > 0)
    return _lookup_snap_ino(mdr);

That condition is weird, in that "snapid" is le64, so this is effectively casting that to u64 (regardless of host endianness), and the if statement just a check to see whether that value != 0. CEPH_NOSNAP, however is defined as (__u64)-2, which will evaluate as being >0 in the above conditional.

It's not clear why this is testing for 0 instead of CEPH_NOSNAP. I guess I need to understand why CEPH_NOSNAP is -2 and not 0. What is the special significance of snapid being 0 in the if condition in Server::handle_client_lookup_ino?