VMM: syscall for reordering pages in vm

[Willibald Krenn]

Quick Summary:

(Good or Bad?) Idea of implementing a syscall that allows
for virtual memory page exchange by modifying the physical<->virtual
page mapping. Intended usage: Moving pages in virtual memory without
the need to copy them. Feedback welcome!



Longer Version:

I need some advice from the Linux VMM-gurus on following idea: Suppose
a user-land program wants to re-order memory it has allocated and
suppose further that this program has done allocations in a way that the
reordering can be done by exchanging whole pages in virtual memory, as
indicated below.

Page5  Program.. (part 2)
Page4  swappedout/"free"
Page3  Program.. (part 3)
Page2  Program.. (part 4)
Page1  swappedout/"free"
Page0  Program.. (part 1)

How about implementing a system call that allows the user-program to
exchange pages? In the example above e.g. Page 5/Page 1 and
Page 3/Page 2.
As far as I understand, the system call would 'just' have to change the
physical<->virtual mapping of these pages and the exchange would be done
without having to copy -in the example above- 4*PageSize bytes in 
userland. (Of course there are some additional benefits in the given 
example, as there is no need to swap in a page as a buffer for the 
copy-operation...)


However, before investigating this idea any further, I figured it would 
be best to ask for comments from the experts here.. (I've barely any 
knowledge about the Linux-VMM and therefore my idea might be complete 
and utter nonsense.)
Any kind of feedback is greatly appreciated!


Thanks for all your time,
 Willibald Krenn

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