It doesn't actually have any semantics: it was never implemented.
I _meant_ for it to be the same kind of callback as "nopage" is, to let
anybody do his own management of copy-on-write behaviour, but nothing
needed it so it was just left as a stub in mm.h.
In short, if you have a use for it, please post your suggested semantics,
and the rationale for it, and I'll try to think about it for a while. But
this really is a case of somebody having to come up with a sane
implementation and be able to make a good case for why it should be
considered the right thing to do..
Linus