netdev - Re: [PATCH net-next] net: mhi-net: Add de-aggeration support

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Message-ID: <CAMZdPi_dMBDafAVoHbqwR9RDbtZSJpGd48oCMmL1qAgR+PCFGQ@mail.gmail.com>
Date:   Mon, 1 Feb 2021 17:48:55 +0100
From:   Loic Poulain <loic.poulain@...aro.org>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        David Miller <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] net: mhi-net: Add de-aggeration support

On Mon, 1 Feb 2021 at 15:24, Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> On Mon, Feb 1, 2021 at 3:08 AM Loic Poulain <loic.poulain@...aro.org> wrote:
> >
> > Hi Jakub, Willem,
> >
> > On Sat, 30 Jan 2021 at 02:01, Jakub Kicinski <kuba@...nel.org> wrote:
> > >
> > > On Mon, 25 Jan 2021 16:45:57 +0100 Loic Poulain wrote:
> > > > When device side MTU is larger than host side MRU, the packets
> > > > (typically rmnet packets) are split over multiple MHI transfers.
> > > > In that case, fragments must be re-aggregated to recover the packet
> > > > before forwarding to upper layer.
> > > >
> > > > A fragmented packet result in -EOVERFLOW MHI transaction status for
> > > > each of its fragments, except the final one. Such transfer was
> > > > previoulsy considered as error and fragments were simply dropped.
> > > >
> > > > This patch implements the aggregation mechanism allowing to recover
> > > > the initial packet. It also prints a warning (once) since this behavior
> > > > usually comes from a misconfiguration of the device (modem).
> > > >
> > > > Signed-off-by: Loic Poulain <loic.poulain@...aro.org>
> > >
> > > > +static struct sk_buff *mhi_net_skb_append(struct mhi_device *mhi_dev,
> > > > +                                       struct sk_buff *skb1,
> > > > +                                       struct sk_buff *skb2)
> > > > +{
> > > > +     struct sk_buff *new_skb;
> > > > +
> > > > +     /* This is the first fragment */
> > > > +     if (!skb1)
> > > > +             return skb2;
> > > > +
> > > > +     /* Expand packet */
> > > > +     new_skb = skb_copy_expand(skb1, 0, skb2->len, GFP_ATOMIC);
> > > > +     dev_kfree_skb_any(skb1);
> > > > +     if (!new_skb)
> > > > +             return skb2;
> > >
> > > I don't get it, if you failed to grow the skb you'll return the next
> > > fragment to the caller? So the frame just lost all of its data up to
> > > where skb2 started? The entire fragment "train" should probably be
> > > dropped at this point.
> >
> > Right, there is no point in keeping the partial packet in that case.
> >
> > >
> > > I think you can just hang the skbs off skb_shinfo(p)->frag_list.
> > >
> > > Willem - is it legal to feed frag_listed skbs into netif_rx()?
> >
> > In QMAP case, the packets will be forwarded to rmnet link, which works
> > with linear skb (no NETIF_F_SG), does the linearization will be
> > properly performed by the net core, in the same way as for xmit path?
>
> What is this path to rmnet, if not the usual xmit path / validate_xmit_skb?

I mean, not sure what to do exactly here, instead of using
skb_copy_expand to re-aggregate data from the different skbs, Jakub
suggests chaining the skbs instead (via 'frag_list' and 'next' pointer
I assume), and to push this chained skb to net core via netif_rx. In
that case, I assume the de-fragmentation/linearization will happen in
the net core, right? If the transported protocol is rmnet, the packet
is supposed to reach the rmnet_rx_handler at some point, but rmnet
only works with standard/linear skbs.

Regards,
Loic