Parameters of dkprintf() should be evaluated even if dkprintf() is
disabled. Because this enables to find expression of parameter obsolete
and to avoid unnecessary compiler warnings such as "unused variable".
implemented:
- Pages can be shared between maps.
- A change made to a map is written to the file, at munmap().
not yet implemented:
- VM operation during page IO execution.
Because page IO is executed with VM's lock.
- Page IO, which does not change a file size with any case.
When munmap() races with truncate(), the file size may be changed
illegally.
