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Sorry, I've had nothing to do with mmap beyond fixing bugs.  
The "access" feature was due to Jay Miller, although I believe 
I checked in his patch.

Martin, I don't understand why you think it's reasonable for 
flush to complain here:  the mmap is open for writing, so 
what's surprising about expecting to be able to flush after a 
write?  Simply that there's no associated file, due to copy-on-
write?  Then user code would have to be acutely aware of how 
an mmap'ed object was opened, just to avoid nuisance 
complaints when they flush after writing.

So that's a third alternative:  alter the implementation to make 
mmap.flush() do nothing when an mmap object was opened 
as copy-on-write.