MAP_COPY_ON_WRITE
MAP_COPY_ON_WRITE | MAP_PREFAULT | MAP_VN_EXEC);
MAP_COPY_ON_WRITE | MAP_PREFAULT | MAP_VN_EXEC);
cow = MAP_COPY_ON_WRITE | MAP_PREFAULT |
(cow & MAP_COPY_ON_WRITE) == 0,
if (cow & MAP_COPY_ON_WRITE)
docow |= MAP_COPY_ON_WRITE;