about get_user_pages function

Hello, I was reading https://lwn.net/Articles/753027/ and it says Problems can arise when the kernel decides to perform writeback on some of the pages brought in with get_user_pages(). The writeback process will write-protect the pages so that user-space cannot modify them until writeback is complete, but it knows nothing about DMA operations started by the driver that called get_user_pages(); that I/O may still be ongoing. Does get_user_pages() start a DMA operation? If yes, is it for the user pages content? Or is it copying the page table? Or is the author talking about the DMA operation by the driver sometime after the get_user_pages operation? Chan Kim