svn commit: r265292 - stable/9/sys/net

[Rick Macklem]

Author: rmacklem
Date: Sat May  3 23:48:26 2014
New Revision: 265292
URL: http://svnweb.freebsd.org/changeset/base/265292

Log:
  MFC: r264630
  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.

Modified:
  stable/9/sys/net/if.c
Directory Properties:
  stable/9/sys/   (props changed)
  stable/9/sys/net/   (props changed)

Modified: stable/9/sys/net/if.c
==============================================================================
--- stable/9/sys/net/if.c	Sat May  3 23:42:00 2014	(r265291)
+++ stable/9/sys/net/if.c	Sat May  3 23:48:26 2014	(r265292)
@@ -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>
@@ -674,7 +675,8 @@ 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__));