diff --git a/mk/config.mk.in b/mk/config.mk.in
index e21abe31086a191e7c6022dada234ab3a61962c0..f616629d4a457e531af1bfd9026199175e7dad2c 100644
--- a/mk/config.mk.in
+++ b/mk/config.mk.in
@@ -347,6 +347,11 @@ NoFibHcOpts = -O
 #
 ################################################################################
 
+# Directory used by GHC (and possibly other tools) for storing
+# temporary files.  If your /tmp isn't big enough, either override
+# this in build.mk or set your environment variable "TMPDIR" to point
+# to somewhere with more space.  (TMPDIR=. is a good choice).
+
 TMPDIR			= /tmp
 
 # FPTOOLS_TOP: the top of the fptools hierarchy, absolute path.