diff --git a/sys/kern/uipc_mbuf.c b/sys/kern/uipc_mbuf.c
index 80c40611203..27ca156c23c 100644
--- a/sys/kern/uipc_mbuf.c
+++ b/sys/kern/uipc_mbuf.c
@@ -560,6 +560,11 @@ bad:
  * Partition an mbuf chain in two pieces, returning the tail --
  * all but the first len0 bytes.  In case of failure, it returns NULL and
  * attempts to restore the chain to its original state.
+ *
+ * Note that the resulting mbufs might be read-only, because the new
+ * mbuf can end up sharing an mbuf cluster with the original mbuf if
+ * the "breaking point" happens to lie within a cluster mbuf. Use the
+ * M_WRITABLE() macro to check for this case.
  */
 struct mbuf *
 m_split(struct mbuf *m0, int len0, int wait)
@@ -609,7 +614,6 @@ extpacket:
 		n->m_flags |= M_EXT;
 		n->m_ext = m->m_ext;
 		MEXT_ADD_REF(m);
-		m->m_ext.ext_size = 0; /* For Accounting XXXXXX danger */
 		n->m_data = m->m_data + len;
 	} else {
 		bcopy(mtod(m, caddr_t) + len, mtod(n, caddr_t), remain);
diff --git a/sys/sys/mbuf.h b/sys/sys/mbuf.h
index 4075a5d2265..5cfb1aebf35 100644
--- a/sys/sys/mbuf.h
+++ b/sys/sys/mbuf.h
@@ -344,6 +344,9 @@ struct mbstat {
 /*
  * Compute the amount of space available
  * before the current start of data in an mbuf.
+ *
+ * The M_WRITABLE() is a temporary, conservative safety measure: the burden
+ * of checking writability of the mbuf data area rests solely with the caller.
  */
 #define	M_LEADINGSPACE(m)						\
 	((m)->m_flags & M_EXT ?						\
@@ -354,10 +357,14 @@ struct mbstat {
 /*
  * 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
+ * of checking writability of the mbuf data area rests solely with the caller.
  */
 #define	M_TRAILINGSPACE(m)						\
-	((m)->m_flags & M_EXT ? (m)->m_ext.ext_buf +			\
-	    (m)->m_ext.ext_size - ((m)->m_data + (m)->m_len) :		\
+	((m)->m_flags & M_EXT ?						\
+	    (M_WRITABLE(m) ? (m)->m_ext.ext_buf + (m)->m_ext.ext_size	\
+		- ((m)->m_data + (m)->m_len) : 0) :			\
 	    &(m)->m_dat[MLEN] - ((m)->m_data + (m)->m_len))
 
 /*