private int subIndexRootPage(long addr, SchemaPageContext cxt)
      throws IOException, MetadataException {
    return getPageInstance(SchemaFile.getPageIndex(addr), cxt).getSubIndex();
  }
Only accessed during write operation and every page returned is due to be modified thus they
are all marked dirty.

@param size size of the expected segment
@return always write locked
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/schemaengine/schemaregion/mtree/impl/pbtree/schemafile/pagemgr/PageManager.java