code: public long getOffsetInPage(long pagePlusOffsetAddress) {
      final long offsetInPage = decodeOffset(pagePlusOffsetAddress);
      if (tungstenMemoryMode == MemoryMode.ON_HEAP) {
        return offsetInPage;
      } else {
        // In off-heap mode, an offset is an absolute address. In encodePageNumberAndOffset, we
        // converted the absolute address into a relative address. Here, we invert that operation:
        final int pageNumber = decodePageNumber(pagePlusOffsetAddress);
        assert (pageNumber >= 0 && pageNumber < PAGE_TABLE_SIZE);
        final MemoryBlock page = pageTable[pageNumber];
        assert (page != null);
        return page.getBaseOffset() + offsetInPage;
      }
    }
docs: Get the offset associated with an address encoded by
  {@link TaskMemoryManager#encodePageNumberAndOffset(MemoryBlock, long)}