long getGarbageChainLeftPage() {
        Debug.$assert0.t(isGarbagePage());
        if (_alloc + GARBAGE_BLOCK_SIZE > _bufferSize)
            return -1;
        long page = getLong(_alloc + GARBAGE_BLOCK_LEFT_PAGE);
        Debug.$assert0.t(page > 0 && page <= MAX_VALID_PAGE_ADDR && page != _page);
        return page;
    }