diff --git a/llvm/test/lit.cfg.py b/llvm/test/lit.cfg.py
index d30cd2002d36c..725ddb877f9ec 100644
--- a/llvm/test/lit.cfg.py
+++ b/llvm/test/lit.cfg.py
@@ -804,6 +804,12 @@ def host_unwind_supports_jit():
     config.available_features.add("use_msan_with_origins")
 
 
+# Restrict the size of the on-disk CAS for tests. This allows testing in
+# constrained environments (e.g. small TMPDIR). It also prevents leaving
+# behind large files on file systems that do not support sparse files if a test
+# crashes before resizing the file.
+config.environment["LLVM_CAS_MAX_MAPPING_SIZE"] = "%d" % (100 * 1024 * 1024)
+
 # Some tools support an environment variable "OBJECT_MODE" on AIX OS, which
 # controls the kind of objects they will support. If there is no "OBJECT_MODE"
 # environment variable specified, the default behaviour is to support 32-bit