diff --git a/Documentation/gitattributes.txt b/Documentation/gitattributes.txt
index e3b1de8033..145dd10967 100644
--- a/Documentation/gitattributes.txt
+++ b/Documentation/gitattributes.txt
@@ -374,6 +374,11 @@ substitution.  For example:
 	smudge = git-p4-filter --smudge %f
 ------------------------
 
+Note that "%f" is the name of the path that is being worked on. Depending
+on the version that is being filtered, the corresponding file on disk may
+not exist, or may have different contents. So, smudge and clean commands
+should not try to access the file on disk, but only act as filters on the
+content provided to them on standard input.
 
 Interaction between checkin/checkout attributes
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^