  With the advent of pretrained language models (LMs), increasing research
efforts have been focusing on infusing commonsense and domain-specific
knowledge to prepare LMs for downstream tasks. These works attempt to leverage
knowledge graphs, the de facto standard of symbolic knowledge representation,
along with pretrained LMs. While existing approaches have leveraged external
knowledge, it remains an open question how to jointly incorporate knowledge
graphs representing varying contexts, from local (e.g., sentence), to
document-level, to global knowledge, to enable knowledge-rich exchange across
these contexts. Such rich contextualization can be especially beneficial for
long document understanding tasks since standard pretrained LMs are typically
bounded by the input sequence length. In light of these challenges, we propose
KALM, a Knowledge-Aware Language Model that jointly leverages knowledge in
local, document-level, and global contexts for long document understanding.
KALM first encodes long documents and knowledge graphs into the three
knowledge-aware context representations. It then processes each context with
context-specific layers, followed by a context fusion layer that facilitates
knowledge exchange to derive an overarching document representation. Extensive
experiments demonstrate that KALM achieves state-of-the-art performance on six
long document understanding tasks and datasets. Further analyses reveal that
the three knowledge-aware contexts are complementary and they all contribute to
model performance, while the importance and information exchange patterns of
different contexts vary with respect to different tasks and datasets.
