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For NFS mounts using rsize,wsize=65536 over TSO enabled

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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@gmail.com, markus.gebert@hostpoint.ch
MFC after:	2 weeks
This commit is contained in:
Rick Macklem 2014-04-17 23:31:50 +00:00
parent edc63b46e0
commit 209579aeac
1 changed files with 3 additions and 1 deletions
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4
sys/net/if.c
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@ -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, int vmove)
#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__));