== A kernel NFS error message explained

Suppose that your machine is an NFS client, and it periodically logs
a kernel error message that looks like:

> ((nfs_stat_to_errno: bad nfs status return value: 16))

What does this mean?

The short summary is that your NFS server is violating the NFS
specifications. NFS requests can fail, and when they do the server
returns an error number to tell the client why. The NFS specs say what
the allowed error numbers are; the Linux kernel client code checks
that the error number it got was one of the allowed ones. You get this
message when this check fails. The number is the (decimal) number of
the invalid NFS error code.

(You can find the list of valid NFS v3 error codes in [[RFC 1813
http://www.ietf.org/rfc/rfc1813.txt]] section 2.5 and the NFS v4 error
codes in [[RFC 3530 http://www.ietf.org/rfc/rfc3530.txt]] section 18.  I
believe that NFS v2 error codes are a subset of NFS v3 ones, and NFS v4
ones are almost a superset of NFS v3 ones.)

Now some guesses as to what those stray error values actually mean.

If you look at the actual numbers of those defined error values and have
a Solaris machine handy (or just a general knowledge of Unix), something
jumps out at you immediately: many of the NFS 'error codes' actually
have the same numbers as the corresponding SunOS _errno_ value for the
problem. I strongly suspect that the original NFS server and client code
did not have NFS 'error codes' as such; instead it took the server's
kernel _errno_ value generated from trying to do the request, stuffed
it in the NFS reply, and on the client took the NFS error code and set
_errno_ to that.

I further suspect that some NFS servers still do this. Thus, if you get
get such an error message from your client kernels, your best bet at
figuring out what the NFS server is trying to say is to look at what
error that value is on your NFS server. Figuring out why some operation
seems to be getting this error is up to you.

(In the specific case of this message for us, our NFS servers are
Solaris 10 machines, where 16 is _EBUSY_. Low _errno_ numbers are
relatively well standardized; high ones are not necessarily so.)