[$] 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 theproblems surrounding get_user_pages() (and friends) and theinteraction with DMA. It is not the first time the topic has come up,there was also a discussion about it at theLinux Storage, Filesystem, and Memory-Management Summit back in April. Ina nutshell, the problem is that multiple parts of the kernel think theyhave responsibility for the same chunk of memory, but they do notcoordinate their activities; as might be guessed, mayhem can sometimes ensue.