page swap out

["Wang Yu" <wangyuict@xxxxxxxxx>]

Hi, all
In <Understanding the Linux Kernel>, there are following descriptions
about swapped-out page:

Because a page may belong to the address spaces of several processes,
it may be swapped out from the address space of one process and still
remain in main memory; therefore, it is possible to swap out the same
page several times. A page is physically swapped out and stored just
once, of course, but each subsequent attempt to swap it out increases
the swap_map counter.

My doubt is that when a page is physically swapped out and stored in
swap area and when a page may have been swapped out from the address
space of one process but still remain in main memory?

Best regards!

-- 
National Research Center for Intelligent Computing Systems
Institute of Computing Technology, Chinese Academy of Sciences

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