From mboxrd@z Thu Jan  1 00:00:00 1970
From: Tom Herbert <tom@herbertland.com>
Subject: Re: [PATCH net v2 1/4] ipv4: no CHECKSUM_PARTIAL on MSG_MORE corked sockets
Date: Tue, 27 Oct 2015 09:41:46 -0700
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Cc: Linux Kernel Network Developers <netdev@vger.kernel.org>,
	Eric Dumazet <edumazet@google.com>,
	Vladislav Yasevich <vyasevich@gmail.com>,
	Benjamin Coddington <bcodding@redhat.com>
To: Hannes Frederic Sowa <hannes@stressinduktion.org>
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On Tue, Oct 27, 2015 at 9:34 AM, Hannes Frederic Sowa
<hannes@stressinduktion.org> wrote:
> On Tue, Oct 27, 2015, at 17:04, Tom Herbert wrote:
>> On Tue, Oct 27, 2015 at 8:02 AM, Hannes Frederic Sowa
>> <hannes@stressinduktion.org> wrote:
>> > We cannot reliable calculate packet size on MSG_MORE corked sockets
>> > and thus cannot decide if they are going to be fragmented later on,
>> > so better not use CHECKSUM_PARTIAL in the first place.
>> >
>> MSG_MORE should be independent of checksum offload. If packet is
>> fragmented the fix in ip_output will ensure that skb_checksum_help is
>> properly called.
>
> The probability is that we are going to fragment if MSG_MORE is set,
> because exceeding link mtu is quite probable, see e.g. NFS use case. Why
> not simply use the csum functions during copy-in in that case? It makes
> much more sense to me.
>
For datagram sockets MSG_MORE means that more datagrams will be sent,
it's not used to incrementally add data to a datagram already queued
(SEQPACKET with EOR is for that).

> I don't see a reason to test for fragment length at all, then.
>
> Bye,
> Hannes