On Mon, 2014-02-10 at 20:31 +0800, Herbert Xu wrote:

On Mon, Feb 10, 2014 at 01:23:31PM +0100, Florian Westphal wrote:

quoted

Herbert Xu [off-list ref] wrote:

quoted

quoted

quoted

static void ip_gso_adjust_seglen(struct sk_buff *skb)
{
        unsigned int mtu;

        if (!skb_is_gso(skb))
                return;

        mtu = ip_dst_mtu_maybe_forward(skb_dst(skb), true);
        skb_shinfo(skb)->gso_size = mtu - sizeof(struct iphdr);
}

But this yields

[   28.644776] kernel BUG at net/net/core/skbuff.c:2984!

Yep, lets CC Herbert Xu, as he 'owns' skb_segment()

IMHO we should just stop merging ~DF packets altogether, at least
for TCP.

Eric, you added DF aggregation in db8caf3dbc77599dc90f4ea0a803cd1d97116f30
(gro: should aggregate frames without DF).

I guess you don't want to revert this commit?
Any other ideas?

skb_gso_segment() is already very complex, I don't want to add more code
to it.  And that seems unavoidable if we need to de-couple nr_frags and
gso_size.

I don't think adding all this complexity just to be able to
aggregate ~DF packets (which are just wrong to begin with) is
worth it.

Wrong by your standards. Which are not universal.

If aggregating ~DF packets was a one-liner then sure, but there
is a reason why I didn't aggregate them in the first place and
you've found it :)

I sent months ago a solution for skb_segment() that you ignored.

I understand you never hit cases where DF was not set, I can tell you
its happening in the real world.

GRO stack already breaks reversibility by definition since day-0

Recent tunneling support breaks it as well.