Would a good guess be:
   
   0 == success
   !0 == failure ( page fault). If so, would the right return value be EFAULT ?


this is right.  in the failure case i see explicit -1, explicit EFAULT
and random-non-zero values (eg, i believe on the alpha it returns the
address of the error-path for kcopy.)


i've updated the kcopy(9) manual.