netdev - Re: [PATCH net-next v2 1/8] udp: fixup csum for GSO receive slow path

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Date:   Mon, 29 Mar 2021 09:52:25 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Alexander Lobakin <alobakin@...me>
Subject: Re: [PATCH net-next v2 1/8] udp: fixup csum for GSO receive slow path

> > > > > Additionally the segmented packets UDP CB still refers to the original
> > > > > GSO packet len. Overall that causes unexpected/wrong csum validation
> > > > > errors later in the UDP receive path.
> > > > >
> > > > > We could possibly address the issue with some additional checks and
> > > > > csum mangling in the UDP tunnel code. Since the issue affects only
> > > > > this UDP receive slow path, let's set a suitable csum status there.
> > > > >
> > > > > v1 -> v2:
> > > > >  - restrict the csum update to the packets strictly needing them
> > > > >  - hopefully clarify the commit message and code comments
> > > > >
> > > > > Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> > > > > +       if (skb->ip_summed == CHECKSUM_NONE && !skb->csum_valid)
> > > > > +               skb->csum_valid = 1;
> > > >
> > > > Not entirely obvious is that UDP packets arriving on a device with rx
> > > > checksum offload off, i.e., with CHECKSUM_NONE, are not matched by
> > > > this test.
> > > >
> > > > I assume that such packets are not coalesced by the GRO layer in the
> > > > first place. But I can't immediately spot the reason for it..
> > >
> > > Packets with CHECKSUM_NONE are actually aggregated by the GRO engine.
> > >
> > > Their checksum is validated by:
> > >
> > > udp4_gro_receive -> skb_gro_checksum_validate_zero_check()
> > >         -> __skb_gro_checksum_validate -> __skb_gro_checksum_validate_complete()
> > >
> > > and skb->ip_summed is changed to CHECKSUM_UNNECESSARY by:
> > >
> > > __skb_gro_checksum_validate -> skb_gro_incr_csum_unnecessary
> > >         -> __skb_incr_checksum_unnecessary()
> > >
> > > and finally to CHECKSUM_PARTIAL by:
> > >
> > > udp4_gro_complete() -> udp_gro_complete() -> udp_gro_complete_segment()
> > >
> > > Do you prefer I resubmit with some more comments, either in the commit
> > > message or in the code?
> >
> > That breaks the checksum-and-copy optimization when delivering to
> > local sockets. I wonder if that is a regression.
>
> The conversion to CHECKSUM_UNNECESSARY happens since
> commit 573e8fca255a27e3573b51f9b183d62641c47a3d.
>
> Even the conversion to CHECKSUM_PARTIAL happens independently from this
> series, since commit 6f1c0ea133a6e4a193a7b285efe209664caeea43.
>
> I don't see a regression here ?!?

I mean that UDP packets with local destination socket and no tunnels
that arrive with CHECKSUM_NONE normally benefit from the
checksum-and-copy optimization in recvmsg() when copying to user.

If those packets are now checksummed during GRO, that voids that
optimization, and the packet payload is now touched twice.