netdev - Re: [PATCH net] ipv6: don't use CHECKSUM_PARTIAL on MSG_MORE/UDP

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Message-Id: <1445431953.1205852.416263057.606CE717@webmail.messagingengine.com>
Date:	Wed, 21 Oct 2015 14:52:33 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Vlad Yasevich <vyasevich@...il.com>, netdev@...r.kernel.org
Cc:	sd@...sysnail.net, bcodding@...hat.com
Subject: Re: [PATCH net] ipv6: don't use CHECKSUM_PARTIAL on MSG_MORE/UDP_CORK
 sockets

On Tue, Oct 20, 2015, at 23:39, Vlad Yasevich wrote:
> I am now wondering if this might be UFO related instead and looking for a
> nic that
> has UFO support.

I doubt that.

We overallocate memory first time in ip6_append_data because we are in
MSG_MORE mode. Then, in my case the second write only copies data into
the first skb on the write queue, no skb is appended to frags_list. So
udp6_hwcsum_outgoing doesn't clean up the flags, either.

We can improve the check if we fragment an ipv6 skb in ip6_append_mode
in net-next, I agree. But I still see this fix suitable for 'net' tree.

We could also improve udp6_hwcsum_outgoing to check if our packets get
fragmented and fall back to the clean-up path. But I think this kind of
optimization should go into net-next, too.

Currently the check made sure we don't use PARTIAL on skbs which could
fragment. MSG_MORE somehow circumvented that check, so I think the fix
is good to go. We certainly can try to improve PARTIAL checksums for
fragmented packets.

What do you think?

Bye,
Hannes
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