On Fri, Mar 05, 2021 at 04:18:36AM +0000, Matthew Wilcox (Oracle) wrote:

Our type system does not currently distinguish between tail pages and
head or single pages.  This is a problem because we call compound_head()
multiple times (and the compiler cannot optimise it out), bloating the
kernel.  It also makes programming hard as it is often unclear whether
a function operates on an individual page, or an entire compound page.

I've pushed a new version out here:
https://git.infradead.org/users/willy/pagecache.git/shortlog/refs/heads/folio

I think it takes into account everyone's comments so far.  It even compiles.