Because they're part of the page cache now, and won't be recycled until newer 
pages 'push' them out of memory. I for one would be very annoyed an mmap()'s 
associated cache was dropped immediately on munmap, there are many instances 
in which it would be reused immediately after (think running two GTK+ apps in 
a row, and having to reload libgtk from disk each time).
-Ryan
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