Why traces like "WARNING: at net/core/dev.c:1907 skb_warn_bad_offload+0xc2/0xf0()" are generated?

Issue

• This happens after we set up the following IP tables configuration:

[root@server1 ~]# iptables -t nat -L
Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination

Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination
MASQUERADE  all  --  192.168.1.0/20        anywhere
MASQUERADE  all  --  192.168.1.0/20        anywhere

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

 [root@server1 ~]# iptables-save
\# Generated by iptables-save v1.4.7 on Tue Mar  3 14:31:42 2015 *nat 
:PREROUTING ACCEPT [557377:268385174] :POSTROUTING ACCEPT 
 [52928:11852616] :OUTPUT ACCEPT [52961:11854332] -A POSTROUTING -s 
192.168.1.0/20 -o ib0 -j MASQUERADE -A POSTROUTING -s 192.168.1.0/20 -o 
eth0 -j MASQUERADE COMMIT # Completed on Tue Mar  3 14:31:42 2015 # 
Generated by iptables-save v1.4.7 on Tue Mar  3 14:31:42 2015 *filter 
:INPUT ACCEPT [1056357:411114535] :FORWARD DROP [29925:1708444] 
:OUTPUT ACCEPT [197411:294534974] :LOGGING - [0:0] -A FORWARD -s 
192.168.1.0/20 -i ib0 -o eth0 -j ACCEPT -A FORWARD -m state --state 
RELATED,ESTABLISHED -j ACCEPT -A FORWARD -j LOG --log-prefix "DROP:" 
--log-level 6 -A LOGGING -j LOG -A LOGGING -j ACCEPT -A LOGGING -j 
DROP -A LOGGING -j LOGGING COMMIT # Completed on Tue Mar  3 14:31:42 
2015

Mar  3 14:30:45 server2 kernel: ------------[ cut here ]------------ 
Mar  3 14:30:45 server2 kernel: WARNING: at net/core/dev.c:1907 skb_warn_bad_offload+0xc2/0xf0() (Tainted: P        W  ---------------   )
Mar  3 14:30:45 server2 kernel: Hardware name: ProLiant XL230a Gen9 
Mar  3 14:30:45 server2 kernel: igb: caps=(0x32114bb3, 0x0) len=71data_len=0 ip_summed=1
Mar  3 14:30:45 server2 kernel: Modules linked in: ipt_LOG xt_state ipt_MASQUERADE iptable_nat nf_nat nf_conntrack_ipv4 nf_conntrack nf_defrag_ipv4 iptable_filter ip_tables lnet_selftest(U) lmv(U) fld(U) mgc(U) lustre(U) lov(U) osc(U) mdc(U) fid(U) ko2iblnd(U) ptlrpc(U)
obdclass(U) lnet(U) lvfs(U) libcfs(U) mmfs26(U) mmfslinux(U) sha512_generic sha256_generic crc32c_intel tracedev(U) ipmi_watchdog ipmi_devintf nfs lockd fscache auth_rpcgss nfs_acl sunrpc mptctl mptbase autofs4 cpufreq_ondemand freq_table pcc_cpufreq rdma_ucm(U) ib_ucm(U) rdma_cm(U) iw_cm(U) ib_ipoib(U) ib_cm(U) ib_uverbs(U) ib_umad(U) mlx5_ib(U) mlx5_core(U) mlx4_en(U) ptp pps_core mlx4_ib(U) ib_sa(U) ib_mad(U) ib_core(U) ib_addr(U) ipv6 mlx4_core(U) compat(U) vfat fat knem(U) microcode iTCO_wdt iTCO_vendor_support power_meter i2c_i801 lpc_ich mfd_core sg hpilo hpwdt igb(U) dca i2c_algo_bit i2c_core ext4 jbd2 mbcache sd_mod crc_t10dif wmi dm_mirror dm_region_hash dm_log dm_mod [last unloaded: libcfs]
Mar  3 14:30:45 server2 kernel: Pid: 0, comm: swapper Tainted: P        W   ---------------    2.6.32-431.el6.x86_64 #1
Mar  3 14:30:45 server2 kernel: Call Trace:
Mar  3 14:30:45 server2 kernel: <IRQ>  [<ffffffff81071e27>] ? warn_slowpath_common+0x87/0xc0
Mar  3 14:30:45 server2 kernel: [<ffffffff81071f16>] ? warn_slowpath_fmt+0x46/0x50
Mar  3 14:30:45 server2 kernel: [<ffffffffa03b1382>] ? igb_get_drvinfo+0x82/0xe0 [igb]
Mar  3 14:30:45 server2 kernel: [<ffffffff8145afd2>] ? skb_warn_bad_offload+0xc2/0xf0
Mar  3 14:30:45 server2 kernel: [<ffffffff814600c1>] ? __skb_gso_segment+0x71/0xc0
Mar  3 14:30:45 server2 kernel: [<ffffffff81460123>] ? skb_gso_segment+0x13/0x20
Mar  3 14:30:45 server2 kernel: [<ffffffff814601cb>] ? dev_hard_start_xmit+0x9b/0x480
Mar  3 14:30:45 server2 kernel: [<ffffffff8147bd0a>] ? sch_direct_xmit+0x15a/0x1c0
Mar  3 14:30:45 server2 kernel: [<ffffffff81460858>] ? dev_queue_xmit+0x228/0x320
Mar  3 14:30:45 server2 kernel: [<ffffffff814666a5>] ? neigh_resolve_output+0x105/0x2d0
Mar  3 14:30:45 server2 kernel: [<ffffffff8149a1c7>] ? ip_finish_output+0x237/0x310
Mar  3 14:30:45 server2 kernel: [<ffffffff8149a358>] ? ip_output+0xb8/0xc0
Mar  3 14:30:45 server2 kernel: [<ffffffff8149562f>] ? ip_forward_finish+0x4f/0x60
Mar  3 14:30:45 server2 kernel: [<ffffffff8149583c>] ? ip_forward+0x1fc/0x430 
Mar  3 14:30:45 server2 kernel: [<ffffffff814939cd>] ? ip_rcv_finish+0x12d/0x440
Mar  3 14:30:45 server2 kernel: [<ffffffff81493f55>] ? ip_rcv+0x275/0x350
Mar  3 14:30:45 server2 kernel: [<ffffffffa04f8b43>] ? mlx4_ib_post_recv+0x283/0x310 [mlx4_ib]
Mar  3 14:30:45 server2 kernel: [<ffffffff8145b54b>] ? __netif_receive_skb+0x4ab/0x750
Mar  3 14:30:45 server2 kernel: [<ffffffff8145f1b8>] ? netif_receive_skb+0x58/0x60
Mar  3 14:30:45 server2 kernel: [<ffffffffa057bc79>] ? ipoib_ib_handle_rx_wc+0x459/0x770 [ib_ipoib]
Mar  3 14:30:45 server2 kernel: [<ffffffffa03f7033>] ? mlx4_eq_int+0xe3/0x1000 [mlx4_core]
Mar  3 14:30:45 server2 kernel: [<ffffffff81059216>] ? enqueue_task+0x66/0x80
Mar  3 14:30:45 server2 kernel: [<ffffffffa057c810>] ? ipoib_poll+0xb0/0x150 [ib_ipoib]
Mar  3 14:30:45 server2 kernel: [<ffffffff81460b43>] ? net_rx_action+0x103/0x2f0
Mar  3 14:30:45 server2 kernel: [<ffffffff81037137>] ? native_apic_msr_write+0x37/0x40
Mar  3 14:30:45 server2 kernel: [<ffffffff8107a8e1>] ? __do_softirq+0xc1/0x1e0
Mar  3 14:30:45 server2 kernel: [<ffffffff810e6ec0>] ? handle_IRQ_event+0x60/0x170
Mar  3 14:30:45 server2 kernel: [<ffffffff8100c30c>] ? call_softirq+0x1c/0x30
Mar  3 14:30:45 server2 kernel: [<ffffffff8100fa75>] ? do_softirq+0x65/0xa0
Mar  3 14:30:45 server2 kernel: [<ffffffff8107a795>] ? irq_exit+0x85/0x90
Mar  3 14:30:45 server2 kernel: [<ffffffff81530fe5>] ? do_IRQ+0x75/0xf0
Mar  3 14:30:45 server2 kernel: [<ffffffff8100b9d3>] ? ret_from_intr+0x0/0x11
Mar  3 14:30:45 server2 kernel: <EOI>  [<ffffffff812e09be>] ? intel_idle+0xde/0x170
Mar  3 14:30:45 server2 kernel: [<ffffffff812e09a1>] ? intel_idle+0xc1/0x170
Mar  3 14:30:45 server2 kernel: [<ffffffff814266f7>] ? cpuidle_idle_call+0xa7/0x140
Mar  3 14:30:45 server2 kernel: [<ffffffff81009fc6>] ? cpu_idle+0xb6/0x110
Mar  3 14:30:45 server2 kernel: [<ffffffff81520e13>] ? start_secondary+0x2ac/0x2ef
Mar  3 14:30:45 server2 kernel: ---[ end trace 95f2bcf1f06224f0 ]---