1. It doesn't seem like generic_xlog.c has thought very carefully about
the semantics of the "hole" between pd_lower and pd_upper. The mainline
XLOG code goes to some lengths to ensure that the hole stays all-zeroes;
for example RestoreBlockImage() explicitly zeroes the hole when restoring
from a full-page image that has a hole. But generic_xlog.c's redo routine
does not do anything comparable, nor does GenericXLogFinish make any
effort to ensure that the "hole" is all-zeroes after normal application of
a generic update. The reason this is of interest is that it means the
contents of the "hole" could diverge between master and slave, or differ
between the original state of a database and what it is after a crash and
recovery. That would at least complicate forensic comparisons of pages,
and I think it might also break checksumming. We thought that this was
important enough to take the trouble of explicitly zeroing holes during
mainline XLOG replay. Shouldn't generic_xlog.c take the same trouble?
2. Unless I'm missing something, contrib/bloom is making no effort
to ensure that bloom index pages can be distinguished from other pages
by pg_filedump. That's fine if your expectation is that bloom will always
be a toy with no use in production; but otherwise, not so much. You need
to make sure that the last two bytes of the page special space contain a
uniquely identifiable code; compare spgist_page_id in SPGiST indexes.
regards, tom lane