On Wed, 2013-11-06 at 17:21 -0800, Eric Dumazet wrote:

On Thu, 2013-11-07 at 02:13 +0100, Hannes Frederic Sowa wrote:

quoted

On Wed, Nov 06, 2013 at 04:15:21PM -0800, Eric Dumazet wrote:

quoted

On Wed, 2013-11-06 at 11:47 -0800, Eric Dumazet wrote:

quoted

I'll try a different way.

The frag_list would contain a bunch of frags, that we logically add to the bunch
of frags found in the first skb shared_info structure.

Here is the patch I came into (I tested it and it works very fine)

The theory is that in GRO stack, all skbs use the head_frag trick,
so even if one NIC pulled some payload into skb->head, we do not have to
copy anything. Outside of GRO stack, we are not supposed to provide data
in skb->head (I am speaking of the skb found on the frag_list extension,
not the skb_head)

I put a fallback code, just in case, with a WARN_ON_ONCE() so that we
can catch the offenders (if any) to fix them.

I renamed @skb to @skb_head to more clearly document this code.
Same for @i renamed to @cur_frag

I wanted to understand this code more closely and tried it with a test case I
used for the UDP_CORK bugs and also for the tbf panic.

The packet is allocated as an UFO one and gets segmented by tbf.

# tc qdisc replace dev eth0 root tbf rate 200kbit latency 20ms burst 5kb
# ./udptest
(Just doing two writes of 200 bytes, then a write of 4096 bytes on a udp
socket. I can send you the source (or a stripped down version, because it got
realy noisy.))

Interesting :

                       if (cskb == head_skb)
                               cskb = skb_shinfo(head_skb)->frag_list;
                       else
                               cskb = cskb->next;
                       if (!cskb) {
                               WARN_ON_ONCE(1);
                               goto err;
                       }

So here either head_skb->frag_list is NULL, or the frag_list chain finishes too early.

More probably I have a bug in the code ;)

Oh yes, I missed a : data_len += remain;

at line 2906 :

	offset = remain;
+	data_len += remain;
	continue;