[$] DMA and get_user_pages()

In the RDMA microconference of the 2018 Linux Plumbers Conference (LPC),
John Hubbard, Dan Williams, and Matthew Wilcox led a discussion on the
problems surrounding get_user_pages() (and friends) and the
interaction with DMA. It is not the first time the topic has come up,
there was also a discussion about it at the
Linux Storage, Filesystem, and Memory-Management Summit back in April. In
a nutshell, the problem is that multiple parts of the kernel think they
have responsibility for the same chunk of memory, but they do not
coordinate their activities; as might be guessed, mayhem can sometimes ensue.