[$] Clarifying memory management with page folios

Memory management generally works at the level of pages, which typicallycontain 4,096 bytes but may be larger. The kernel, though, has extendedthe concept of pages to include compound pages, which are groups ofcontiguous single pages. That, in turn, has made the definition ofwhat a "page" is a bit fuzzy. Matthew Wilcox has been working since lastyear on a concept called "page folios" which is meant to bring the pictureback into focus; whether the memory-management community will accept itremains unclear, though.