svn commit: r264630 - head/sys/net

[Rick Macklem]

Author: rmacklem
Date: Thu Apr 17 23:31:50 2014
New Revision: 264630
URL: http://svnweb.freebsd.org/changeset/base/264630

Log:
  For NFS mounts using rsize,wsize=65536 over TSO enabled
  network interfaces limited to 32 transmit segments, there
  are two known issues.
  The more serious one is that for an I/O of slightly less than 64K,
  the net device driver prepends an ethernet header, resulting in a
  TSO segment slightly larger than 64K. Since m_defrag() copies this
  into 33 mbuf clusters, the transmit fails with EFBIG.
  A tester indicated observing a similar failure using iSCSI.
  
  The second less critical problem is that the network
  device driver must copy the mbuf chain via m_defrag()
  (m_collapse() is not sufficient), resulting in measurable overhead.
  
  This patch reduces the default size of if_hw_tsomax
  slightly, so that the first issue is avoided.
  Fixing the second issue will require a way for the
  network device driver to inform tcp_output() that it
  is limited to 32 transmit segments.
  
  Reported and tested by:	csforgeron at gmail.com, markus.gebert at hostpoint.ch
  MFC after:	2 weeks

Modified:
  head/sys/net/if.c

Modified: head/sys/net/if.c
==============================================================================
--- head/sys/net/if.c	Thu Apr 17 21:43:34 2014	(r264629)
+++ head/sys/net/if.c	Thu Apr 17 23:31:50 2014	(r264630)
@@ -74,6 +74,7 @@
 #include <net/vnet.h>
 
 #if defined(INET) || defined(INET6)
+#include <net/ethernet.h>
 #include <netinet/in.h>
 #include <netinet/in_var.h>
 #include <netinet/ip.h>
@@ -655,7 +656,8 @@ if_attach_internal(struct ifnet *ifp, in
 #if defined(INET) || defined(INET6)
 		/* Initialize to max value. */
 		if (ifp->if_hw_tsomax == 0)
-			ifp->if_hw_tsomax = IP_MAXPACKET;
+			ifp->if_hw_tsomax = min(IP_MAXPACKET, 32 * MCLBYTES -
+			    (ETHER_HDR_LEN + ETHER_VLAN_ENCAP_LEN));
 		KASSERT(ifp->if_hw_tsomax <= IP_MAXPACKET &&
 		    ifp->if_hw_tsomax >= IP_MAXPACKET / 8,
 		    ("%s: tsomax outside of range", __func__));