Network stack returning EFBIG?

[Rick Macklem]

Markus Gebert wrote:
> 
> On 20.03.2014, at 14:51, wollman at bimajority.org wrote:
> 
> > In article <21290.60558.750106.630804 at hergotha.csail.mit.edu>, I
> > wrote:
> > 
> >> Since we put this server into production, random network system
> >> calls
> >> have started failing with [EFBIG] or maybe sometimes [EIO].  I've
> >> observed this with a simple ping, but various daemons also log the
> >> errors:
> >> Mar 20 09:22:04 nfs-prod-4 sshd[42487]: fatal: Write failed: File
> >> too
> >> large [preauth]
> >> Mar 20 09:23:44 nfs-prod-4 nrpe[42492]: Error: Could not complete
> >> SSL
> >> handshake. 5
> > 
> > I found at least one call stack where this happens and it does get
> > returned all the way to userspace:
> > 
> > 17  15547   _bus_dmamap_load_buffer:return
> >              kernel`_bus_dmamap_load_mbuf_sg+0x5f
> >              kernel`bus_dmamap_load_mbuf_sg+0x38
> >              kernel`ixgbe_xmit+0xcf
> >              kernel`ixgbe_mq_start_locked+0x94
> >              kernel`ixgbe_mq_start+0x12a
> >              if_lagg.ko`lagg_transmit+0xc4
> >              kernel`ether_output_frame+0x33
> >              kernel`ether_output+0x4fe
> >              kernel`ip_output+0xd74
> >              kernel`tcp_output+0xfea
> >              kernel`tcp_usr_send+0x325
> >              kernel`sosend_generic+0x3f6
> >              kernel`soo_write+0x5e
> >              kernel`dofilewrite+0x85
> >              kernel`kern_writev+0x6c
> >              kernel`sys_write+0x64
> >              kernel`amd64_syscall+0x5ea
> >              kernel`0xffffffff808443c7
> 
> This looks pretty similar to what we’ve seen when we got EFBIG:
> 
>  3  28502   _bus_dmamap_load_buffer:return
>               kernel`_bus_dmamap_load_mbuf_sg+0x5f
>               kernel`bus_dmamap_load_mbuf_sg+0x38
>               kernel`ixgbe_xmit+0xcf
>               kernel`ixgbe_mq_start_locked+0x94
>               kernel`ixgbe_mq_start+0x12a
>               kernel`ether_output_frame+0x33
>               kernel`ether_output+0x4fe
>               kernel`ip_output+0xd74
>               kernel`rip_output+0x229
>               kernel`sosend_generic+0x3f6
>               kernel`kern_sendit+0x1a3
>               kernel`sendit+0xdc
>               kernel`sys_sendto+0x4d
>               kernel`amd64_syscall+0x5ea
>               kernel`0xffffffff80d35667
> 
> In our case it looks like some of the ixgbe tx queues get stuck, and
> some don’t. You can test, wether your server shows the same symptoms
> with this command:
> 
> # for CPU in {0..7}; do echo "CPU${CPU}"; cpuset -l ${CPU} ping -i
> 0.5 -c 2 -W 1 10.0.0.1 | grep sendto; done
> 
> We also use 82599EB based ixgbe controllers on affected systems.
> 
> Also see these two threads on freebsd-net:
> 
> http://lists.freebsd.org/pipermail/freebsd-net/2014-February/037967.html
> http://lists.freebsd.org/pipermail/freebsd-net/2014-March/038061.html
> 
> I have started the second one, and there are some more details of
> what we were seeing in case you’re interested.
> 
> Then there is:
> 
> http://www.freebsd.org/cgi/query-pr.cgi?pr=183390
> and:
> https://bugs.freenas.org/issues/4560
> 
Well, the "before" printf() from my patch is indicating a packet > 65535
and that will definitely result in a EFBIG. (There is no way that m_defrag()
can squeeze > 64K into 32 MCLBYTES mbufs.)

Note that the EFBIG will be returned by the call that dequeues this packet
and tries to transmit it (not necessarily the one that generated/queued the
packet). This was pointed out by Ryan in a previous discussion of this.

The code snippet from sys/netinet/tcp_output.c looks pretty straightforward:
       /*
772 	* Limit a burst to t_tsomax minus IP,
773 	* TCP and options length to keep ip->ip_len
774 	* from overflowing or exceeding the maximum
775 	* length allowed by the network interface.
776 	*/
777 	if (len > tp->t_tsomax - hdrlen) {
778 	   len = tp->t_tsomax - hdrlen;
779 	   sendalot = 1;
780 	}
If it is a TSO segment of > 65535, at a glance it would seem that this "if"
is busted. Just to see, you could try replacing line# 777-778 with
       if (len > IP_MAXPACKET - hdrlen) {
           len = IP_MAXPACKET - hdrlen;
which was what it was in 9.1. (Maybe t_tsomax isn't set correctly or somehow
screws up the calculation?

rick

> 
> Markus
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