Repurposing page->mapping

Sounds like a perfect fit for 'struct page' then.

I actually think this is a good idea - have an 'extension' structure for pages to store details that are only used transiently or only on a tiny fraction of pages.
I would re-use private rather than mapping (sorry Chris) and the extension structure would contain:
- refcount
- private
- link back to the struct page
- anything else anyone wanted (flags,wait-queue-heads, linked list head...)
Any code that wanted to could add a page_extension at any time, and current users of private would need to be careful to get/update the right field. A couple of macros would make this easy.

This would have the added benefit that we would have to review all users of ->private, and we could use that opportunity to rename it to something grepable (pg_private) and change it from an unsigned long to a void star. That alone would make it all worth while.