When a process accesses a page not in the main memory but in the swap space, a page fault happens, and the pager reads a page from swap space to main memory.
Is the read a move or copy? In other words, is there still a copy of the page in the swap space?
If it is a copy, does it mean that the pager has to update the copy in the swap space whenever there is a change to the copy of the page in the main space?
Operating System Concepts says:
We ﬁnd a free frame (by taking one from the free-frame list, for example).
We schedule a disk operation to read the desired page into the newly allocated frame.
and this figure also shows copies of some pages (A, C and F) are in both swap space and main memory, and no page only exists in main memory but not in swap space: