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