diff --git a/net/9p/client.c b/net/9p/client.c
index 622ec6a586ee..6c2a768a6ab1 100644
--- a/net/9p/client.c
+++ b/net/9p/client.c
@@ -28,7 +28,11 @@
 #define CREATE_TRACE_POINTS
 #include <trace/events/9p.h>
 
-#define DEFAULT_MSIZE (128 * 1024)
+/* DEFAULT MSIZE = 32 pages worth of payload + P9_HDRSZ +
+ * room for write (16 extra) or read (11 extra) operands.
+ */
+
+#define DEFAULT_MSIZE ((128 * 1024) + P9_IOHDRSZ)
 
 /* Client Option Parsing (code inspired by NFS code)
  *  - a little lazy - parse all client options