[net,v2,1/4] ipv4: no CHECKSUM_PARTIAL on MSG_MORE corked sockets

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.

Cc: Eric Dumazet <edumazet@google.com>
Cc: Vlad Yasevich <vyasevich@gmail.com>
Cc: Benjamin Coddington <bcodding@redhat.com>
Cc: Tom Herbert <tom@herbertland.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
---
 net/ipv4/ip_output.c | 1 +
 1 file changed, 1 insertion(+)

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.

> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Vlad Yasevich <vyasevich@gmail.com>
> Cc: Benjamin Coddington <bcodding@redhat.com>
> Cc: Tom Herbert <tom@herbertland.com>
> Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
> ---
>  net/ipv4/ip_output.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> index 50e2973..0b02417 100644
> --- a/net/ipv4/ip_output.c
> +++ b/net/ipv4/ip_output.c
> @@ -911,6 +911,7 @@ static int __ip_append_data(struct sock *sk,
>         if (transhdrlen &&
>             length + fragheaderlen <= mtu &&
>             rt->dst.dev->features & NETIF_F_V4_CSUM &&
> +           !(flags & MSG_MORE) &&
>             !exthdrlen)
>                 csummode = CHECKSUM_PARTIAL;
>
> --
> 2.5.0
>
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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.

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

Bye,
Hannes
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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
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