I don't think there is need of superuser concept here. What I'm
imagining, again, without having seen the source code, is something
like this:
The file system is formatted and data representing the structure of
the file system are stored normally.
At execution time, the file server check a flag stored in some
place to see if the system is full. If not, the data is changed so
the blocks are "hidden" and execution continues.
If the system is full, the file system starts a console session
presenting the last file edited. At the end of the session, if the
number of free blocks is bigger than the reserved blocks, the blocks are
"hidden" and execution continues. If not, the console session gives
an error and continues.
The flag can be turn on at the same place the error of file system
full is triggered.
In a multi-user environment you can make the file system do something
similar when the system is full, but instead of starting a console
session, delete the last file modified and presenting the user
an error.
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