On Fri, Mar 05, 2021 at 04:18:37AM +0000, Matthew Wilcox (Oracle) wrote:

+/*
+ * A struct folio is either a base (order-0) page or the head page of
+ * a compound page.
+ */

Hmm.  While that comment seems to be true I'm not sure it is the
essence.  Maybe it should be more framed in terms of

"A folio represents a contigously allocated chunk of memory.."

and then extend it with the categories of state and operations performed
on the folio while those get added.  The above statement can still
remain as a low-level explanation, maybe moved to the page member
instead of the type itself.