[$] Repurposing page->mapping

The page structure is one of themost complex in the kernel due to the need to cram the maximum amount ofinformation into as little space as possible. Each field is so heavilyoverloaded that developers prefer to avoid making changes to structpage if they can avoid it. That didn't deter Ji(C)rime Glisse from proposing a significant changeduring two plenary sessions atthe 2018 Linux Storage, Filesystem, and Memory-Management Summit, though.There are some interesting benefits on offer, but getting there will not bea simple task.