Subject: skb_warn_bad_offload warnings with FreeBSD guests

From: Brian Rak <[email-masked]>
Date: Fri, 22 Aug 2014 12:19:10 -0400

We have a number of machines running qemu with bridged networking. We 
have noticed that *sometimes* FreeBSD guests cause this warning to flood 
the host "WARNING: CPU: 5 PID: 3705 at net/core/dev.c:2238 
skb_warn_bad_offload+0xc3/0xd0()".  I haven't been able to come up with 
any sort of reproduction steps, it just seems to happen to some FreeBSD 
guests, but not others.

A full stack trace looks like this:

------------[ cut here ]------------
WARNING: CPU: 1 PID: 7147 at net/core/dev.c:2233 
skb_warn_bad_offload+0xc3/0xd0()
igb: caps=(0x0000000190114bb3, 0x0000000000000000) len=2962 data_len=0 
gso_size=1448 gso_type=5 ip_summed=0
Modules linked in: dm_snapshot dm_bufio ipmi_devintf xt_physdev ebt_arp 
ebt_ip ebtable_nat ebtables cls_fw sch_sfq sch_htb tun kvm_intel kvm 
8021q garp nfnetlink_queue nfnetlink_log nfnetlink bluetooth rfkill 
bridge stp llc xt_CHECKSUM iptable_mangle ipt_REJECT iptable_filter ip
_tables ip6t_REJECT ip6table_filter ip6_tables ipv6 iTCO_wdt 
iTCO_vendor_support ipmi_si ipmi_msghandler microcode pcspkr i2c_i801 
joydev sg lpc_ich shpchp igb dca ptp pps_core hwmon ext4 jbd2 mbcache 
sd_mod crc_t10dif crct10dif_common video ahci libahci xhci_hcd ast ttm 
drm_kms
_helper sysimgblt sysfillrect syscopyarea dm_mirror dm_region_hash 
dm_log dm_mod
CPU: 1 PID: 7147 Comm: qemu-kvm Tainted: G        W 
3.15.5-1.el6.elrepo.x86_64 #1
Hardware name: Supermicro X10SLE-F/HF/X10SLE, BIOS 1.1 07/19/2013
  00000000000008b9 ffff88081fc435d8 ffffffff8163ba90 00000000000008b9
  ffff88081fc43628 ffff88081fc43618 ffffffff8106c30c ffffc90007a06e30
  0000000000000000 ffff8807f2b64000 ffff8807f2b64000 0000000000000000
Call Trace:
  <IRQ>  [<ffffffff8163ba90>] dump_stack+0x49/0x61
  [<ffffffff8106c30c>] warn_slowpath_common+0x8c/0xc0
  [<ffffffff8106c3f6>] warn_slowpath_fmt+0x46/0x50
  [<ffffffff8156ce93>] skb_warn_bad_offload+0xc3/0xd0
  [<ffffffff81574a29>] ? dev_hard_start_xmit+0x339/0x640
  [<ffffffff81574699>] __skb_gso_segment+0x89/0xe0
  [<ffffffff81574876>] dev_hard_start_xmit+0x186/0x640
  [<ffffffff81594f5a>] sch_direct_xmit+0xfa/0x1d0
  [<ffffffff81574f2f>] __dev_queue_xmit+0x1ff/0x4f0
  [<ffffffff81575240>] dev_queue_xmit+0x10/0x20
  [<ffffffffa02e6612>] br_dev_queue_push_xmit+0x82/0xb0 [bridge]
  [<ffffffffa02ee680>] br_nf_dev_queue_xmit+0x20/0x90 [bridge]
  [<ffffffffa02ef4b8>] br_nf_post_routing+0x2d8/0x300 [bridge]
  [<ffffffffa02e6590>] ? deliver_clone+0x60/0x60 [bridge]
  [<ffffffff815a357e>] nf_iterate+0x8e/0xc0
  [<ffffffffa02e6590>] ? deliver_clone+0x60/0x60 [bridge]
  [<ffffffff815a37ad>] nf_hook_slow+0x7d/0x150
  [<ffffffffa02e6590>] ? deliver_clone+0x60/0x60 [bridge]
  [<ffffffffa02ee6f0>] ? br_nf_dev_queue_xmit+0x90/0x90 [bridge]
  [<ffffffffa02e6b43>] br_forward_finish+0x43/0x60 [bridge]
  [<ffffffffa02ee8a8>] br_nf_forward_finish+0x1b8/0x1d0 [bridge]
  [<ffffffffa02ef178>] br_nf_forward_ip+0x3a8/0x410 [bridge]
  [<ffffffffa02e6b00>] ? br_flood_deliver+0x20/0x20 [bridge]
  [<ffffffff815a357e>] nf_iterate+0x8e/0xc0
  [<ffffffffa02e6b00>] ? br_flood_deliver+0x20/0x20 [bridge]
  [<ffffffff815a37ad>] nf_hook_slow+0x7d/0x150
  [<ffffffffa02e6b00>] ? br_flood_deliver+0x20/0x20 [bridge]
  [<ffffffffa02e66e4>] __br_forward+0xa4/0x100 [bridge]
  [<ffffffffa02e7800>] ? NF_HOOK.clone.0+0x70/0x70 [bridge]
  [<ffffffffa02e67d6>] br_forward+0x96/0xb0 [bridge]
  [<ffffffffa02e7800>] ? NF_HOOK.clone.0+0x70/0x70 [bridge]
  [<ffffffffa02e7997>] br_handle_frame_finish+0x197/0x3f0 [bridge]
  [<ffffffffa02e7800>] ? NF_HOOK.clone.0+0x70/0x70 [bridge]
  [<ffffffffa02ef790>] br_nf_pre_routing_finish+0x2b0/0x370 [bridge]
  [<ffffffffa02ef4e0>] ? br_nf_post_routing+0x300/0x300 [bridge]
  [<ffffffffa02ed986>] NF_HOOK_THRESH+0x56/0x60 [bridge]
  [<ffffffffa02eed2b>] br_nf_pre_routing+0x2fb/0x3a0 [bridge]
  [<ffffffff815a357e>] nf_iterate+0x8e/0xc0
  [<ffffffffa02e7800>] ? NF_HOOK.clone.0+0x70/0x70 [bridge]
  [<ffffffff815a37ad>] nf_hook_slow+0x7d/0x150
  [<ffffffffa02e7800>] ? NF_HOOK.clone.0+0x70/0x70 [bridge]
  [<ffffffffa02e7d8c>] br_handle_frame+0x19c/0x240 [bridge]
  [<ffffffffa02e7bf0>] ? br_handle_frame_finish+0x3f0/0x3f0 [bridge]
  [<ffffffff81572fa5>] __netif_receive_skb_core+0x1e5/0x620
  [<ffffffff81573407>] __netif_receive_skb+0x27/0x70
  [<ffffffff81573553>] process_backlog+0x103/0x200
  [<ffffffff81573d62>] net_rx_action+0x112/0x2a0
  [<ffffffff8107111c>] __do_softirq+0xfc/0x2b0
  [<ffffffff810713cd>] ? irq_exit+0xad/0xd0
  [<ffffffff8164a81c>] do_softirq_own_stack+0x1c/0x30
  <EOI>  [<ffffffff81070e75>] do_softirq+0x55/0x60
  [<ffffffff81571e19>] netif_rx_ni+0x39/0x70
  [<ffffffffa03e84e0>] tun_get_user+0x310/0x6c0 [tun]
  [<ffffffffa03e8995>] tun_chr_aio_write+0x85/0xa0 [tun]
  [<ffffffff811beb9d>] do_sync_readv_writev+0x4d/0x80
  [<ffffffff811c0128>] do_readv_writev+0xc8/0x2c0
  [<ffffffff811bebd0>] ? do_sync_readv_writev+0x80/0x80
  [<ffffffff811d2c45>] ? poll_select_set_timeout+0x95/0xb0
  [<ffffffff811c0357>] vfs_writev+0x37/0x50
  [<ffffffff811c0496>] SyS_writev+0x56/0xf0
  [<ffffffff81648ee9>] system_call_fastpath+0x16/0x1b
---[ end trace d26e70ba037ab631 ]---

gso_type=5 and ip_summed=0 are always the same (though len, data_len, 
and gso_size vary).

What is causing this?  I've tried kernels as new as 3.15.5-1, which do 
not appear to help.