netdev - Re: [PATCH net v2 1/4] ipv4: no CHECKSUM_PARTIAL on MSG

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Message-Id: <1445963644.1412577.421660985.2D10CC9A@webmail.messagingengine.com>
Date:	Tue, 27 Oct 2015 17:34:04 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Tom Herbert <tom@...bertland.com>
Cc:	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Vladislav Yasevich <vyasevich@...il.com>,
	Benjamin Coddington <bcodding@...hat.com>
Subject: Re: [PATCH net v2 1/4] ipv4: no CHECKSUM_PARTIAL on MSG_MORE corked
 sockets

On Tue, Oct 27, 2015, at 17:04, Tom Herbert wrote:
> On Tue, Oct 27, 2015 at 8:02 AM, Hannes Frederic Sowa
> <hannes@...essinduktion.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.

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

Bye,
Hannes
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