mmaped files and swap

[Fredrik]

I've got a question regarding understanding the recomendation to disable 
swap.
Since Cassandra uses mlockall to lock the heap in RAM what is the reason 
for disabling swap?
My guess is that is has to do with memory mapped files but as of my 
understanding, accessing pages of
memory mapped files, those pages are never put in swap since they're 
backed by files on disk and the OS
writes those pages to the memory mapped file instead of swap.
We've seen on Cassandra installations on Linux with swap enabled that 
parts of the java process is swaped out and increasing.
So what's swaped out?
Regards
/Fredrik