InnoDB buffer pool active pages

Question

I'd like to optimize (lower) the InnoDB buffer pool size. How can one find out how many distinct pages are active in the InnoDB buffer pool? A page could be made young twice in a while (see SHOW INNODB ENGINE STATUS), so one can't find out how many distinct pages are actually active in the buffer pool.

I have Googled around and read various "how to set/find out optimal innodb buffer size" type Q&As, but they don't involve that question.