As the README for xlog says: There're two kinds of WAL records:
* WAL record that contains enough information to re-generate the entire
contents of a page;
during recovery of these records, blocks are read with:buffer = XLogReadBuffer(reln, blkno, true);
so it can be sure that the block will be read successfully, i.e. buffer
won't be InvalidBuffer
* WAL record provides only enough information to incrementally update the
page;
As all blocks related to these xlog records will be backed up in the xlog
during the first write after checkpoint, and RestoreBkpBlocks will also read
it in with :buffer = XLogReadBuffer(reln, blkno, true);
so it also can be sure that these blocks will be read successfully: because
the fisrt read of it must be in RestoreBkpBlocks, which will reconstruct the
block validly, and none problems in later read.
Then if we come to the path "log_invalid_page", can I say there must be sth
wrong, and we should PANIC?