[$] Explicit pinning of user-space pages

The saga of get_user_pages() - and the problems it causes withinthe kernel - has been extensively chronicled here; see the LWN kernelindex for the full series. In short, get_user_pages() is usedto pin user-space pages in memory for some sort of manipulation outside ofthe owning process(es); that manipulation can sometimes surprise otherparts of the kernel that think they have exclusive rights to the pages inquestion. Thispatch series from John Hubbard does not solve all of the problems, butit does create some infrastructure that may make a solution easier to comeby.