Fun-fact: In the POSIX model you cannot write into a private mapping created through a FD with the writable bit not set. (Or, more precisely, Linux won't let yo

Fun-fact: In the POSIX model you cannot write into a private mapping created through a FD with the writable bit not set. (Or, more precisely, Linux won't let you set the write bit of the mapping). Even though that would be a rather useful thing to have, and wouldn't be hindered by hardware or kernel (which already does CoW of pages).