1 files changed, 11 insertions, 3 deletions

diff --git a/sys/sys/mbuf.h b/sys/sys/mbuf.h
index a6a28291123d..4798c9c2a9ab 100644
--- a/sys/sys/mbuf.h
+++ b/sys/sys/mbuf.h
@@ -1236,6 +1236,16 @@ m_align(struct mbuf *m, int len)
 	(M_WRITABLE(m) ? ((m)->m_data - M_START(m)) : 0)
 
 /*
+ * So M_TRAILINGROOM() is for when you want to know how much space
+ * would be there if it was writable. This can be used to
+ * detect changes in mbufs by knowing the value at one point
+ * and then being able to compare it later to the current M_TRAILINGROOM().
+ * The TRAILINGSPACE() macro is not suitable for this since an mbuf
+ * at one point might not be writable and then later it becomes writable
+ * even though the space at the back of it has not changed.
+ */
+#define M_TRAILINGROOM(m) ((M_START(m) + M_SIZE(m)) - ((m)->m_data + (m)->m_len))
+/*
  * Compute the amount of space available after the end of data in an mbuf.
  *
  * The M_WRITABLE() is a temporary, conservative safety measure: the burden
@@ -1245,9 +1255,7 @@ m_align(struct mbuf *m, int len)
  * for mbufs with external storage.  We now allow mbuf-embedded data to be
  * read-only as well.
  */
-#define	M_TRAILINGSPACE(m)						\
-	(M_WRITABLE(m) ?						\
-	    ((M_START(m) + M_SIZE(m)) - ((m)->m_data + (m)->m_len)) : 0)
+#define	M_TRAILINGSPACE(m) (M_WRITABLE(m) ? M_TRAILINGROOM(m) : 0)
 
 /*
  * Arrange to prepend space of size plen to mbuf m.  If a new mbuf must be