Re: [PATCH 3/4 v2 net-next] net: make GRO aware of skb->head_frag

[Eric Dumazet <eric.dumazet@gmail.com>]

On Tue, 2012-05-01 at 16:10 -0700, Alexander Duyck wrote:
> On 05/01/2012 03:58 PM, Alexander Duyck wrote:

> > Eric,
> >
> > I think I have found a bug, although it is not specific to this patch
> > but it is related.  It looks like the TCP coalesce code is causing
> > tcpdump to fail when using frags.  Based on the comments in the patch I
> > am assuming you have an ixgbe adapter to test with as that is what I
> > reproduced this on.  To reproduce the issue all you need to do is run
> > "tcpdump -i ethX > /dev/null" on one console, and on a second console
> > run a netperf TCP_MAERTS test to some other server.  Tcpdump will exit
> > out with a message about bad address like this:
> >
> > tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
> > listening on eth1, link-type EN10MB (Ethernet), capture size 96 bytes
> > tcpdump: pcap_loop: recvfrom: Bad address
> > 682 packets captured
> > 2357 packets received by filter
> > 1543 packets dropped by kernel
> >
> >
> > A bisect of the issue tracked it down to:
> >
> > 1402d366019fedaa2b024f2bac06b7cc9a8782e1 is first bad commit
> > commit 1402d366019fedaa2b024f2bac06b7cc9a8782e1
> > Author: Eric Dumazet <edumazet@google.com>
> > Date:   Mon Apr 23 07:11:42 2012 +0000
> >
> >     tcp: introduce tcp_try_coalesce
> >     
> >     commit c8628155ece3 (tcp: reduce out_of_order memory use) took care of
> >     coalescing tcp segments provided by legacy devices (linear skbs)
> >     
> >     We extend this idea to fragged skbs, as their truesize can be heavy.
> >     
> >     ixgbe for example uses 256+1024+PAGE_SIZE/2 = 3328 bytes per segment.
> >     
> >     Use this coalescing strategy for receive queue too.
> >     
> >     This contributes to reduce number of tcp collapses, at minimal cost, and
> >     reduces memory overhead and packets drops.
> >     
> >     Signed-off-by: Eric Dumazet <edumazet@google.com>
> >     Cc: Neal Cardwell <ncardwell@google.com>
> >     Cc: Tom Herbert <therbert@google.com>
> >     Cc: Maciej Żenczykowski <maze@google.com>
> >     Cc: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
> >     Acked-by: Neal Cardwell <ncardwell@google.com>
> >     Signed-off-by: David S. Miller <davem@davemloft.net>
> >
> > :040000 040000 8ca3e0b4e6c6a8f375fd800069d24203880623f3 2576d34c5c9cfc717a11e2ebe054143956716b93 M	net
> >
> > I suspect we are dealing with either a shared or cloned skb in this
> > case, though I haven't verified which it is yet.
> >
> > Thanks,
> >
> > Alex
> >
> One additional note.  It looks like LRO and GRO need to be disabled to
> trigger the bug.  If either of them are enabled it doesn't seem to
> occur.  Likely due to the fact that they are doing the coalescing before
> it gets up to the tcp_try_coalesce call.
> 

Thanks Alex, I'll take a look.

It seems my tcpdump is different than yours.