|Subject:||some other memory considerations.|
|Date:||Tue, 26 Oct 2004 02:24:57 -0400|
|User-agent:||Mozilla Thunderbird 0.8 (Macintosh/20040913)|
How will copy-on-write from untrusted-task to untrusted-task work? For example, let's say a client mmap()s a file from some arbitrary translator. To do something like copy-on-write (instead of memcpy three times in some way using containers) the translator would have to request to physmem that it maps that certain page to the client task.
This would be a lot speedier than copy, copy, copy using a container, and follows the current convention of not allowing untrusted tasks to directly map to each other. So this should really should be considered when working out the details of the memory system.
Complications that arise with copy-on-write through physmem would be if the translator that actually owns the page is killed somehow, then the task server would be required to notify the physmem server to actually copy whatever pages are copy-on-write from that translator to the client tasks.
Another complication is how would a copy-on-write page be classified to a client task? Would it be considered owned by it and count toward it's total page count, or will physmem not count it, unless it actually gets copied. In the case that a copy-on-write page is suddenly copied into the client task, but the client task has already exceeded its allowed number of pages and the physmem server is doing an audit, what would happen? Would it count toward a guaranteed frame or extra?
By the way, is it even possible for two arbitrary tasks in L4 to map memory to each other, assuming not one of them are pagers to the other? I'm not entirely sure but I don't think so.
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