A quick and dirty hack to report the blocks of a PostgreSQL data directory
that are currently in the operating system page cache.  The output shows the
database OID, relfilenode, block number and number of OS pages (1 or 2, since
only half an 8KB page might be present).

Currently OS page size is assumed to be the traditional 4KB, likewise 8KB for
PG, and tablespaces and non-main forks are ignored.

Here's a way to see how many pages are double buffered, that is, occupying
both PG and OS memory:

create temp table kernelcache
  (reldatabase oid, relfilenode oid, relblocknumber int, pages int);

copy kernelcache from program '/path/to/pgdata_mincore /path/to/pgdata';

select count(*)
 from pg_buffercache
 join kernelcache using (reldatabase, relfilenode, relblocknumber);