netdev - Re: [PATCH net-next 3/4] net: Support GRO/GSO fraglist chaining.

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Date:   Thu, 19 Dec 2019 11:28:34 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Steffen Klassert <steffen.klassert@...unet.com>
Cc:     David Miller <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>,
        Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 3/4] net: Support GRO/GSO fraglist chaining.

On Thu, Dec 19, 2019 at 3:22 AM Steffen Klassert
<steffen.klassert@...unet.com> wrote:
>
> On Wed, Dec 18, 2019 at 11:02:39AM -0500, Willem de Bruijn wrote:
> > On Wed, Dec 18, 2019 at 8:35 AM Steffen Klassert
> > <steffen.klassert@...unet.com> wrote:
> >
> > > +struct sk_buff *skb_segment_list(struct sk_buff *skb,
> > > +                                netdev_features_t features,
> > > +                                unsigned int offset)
> > > +{
> > > +       struct sk_buff *list_skb = skb_shinfo(skb)->frag_list;
> > > +       unsigned int tnl_hlen = skb_tnl_header_len(skb);
> > > +       unsigned int delta_truesize = 0;
> > > +       unsigned int delta_len = 0;
> > > +       struct sk_buff *tail = NULL;
> > > +       struct sk_buff *nskb;
> > > +
> > > +       skb_push(skb, -skb_network_offset(skb) + offset);
> > > +
> > > +       skb_shinfo(skb)->frag_list = NULL;
> > > +
> > > +       do {
> > > +               nskb = list_skb;
> > > +               list_skb = list_skb->next;
> > > +
> > > +               if (!tail)
> > > +                       skb->next = nskb;
> > > +               else
> > > +                       tail->next = nskb;
> > > +
> > > +               tail = nskb;
> > > +
> > > +               delta_len += nskb->len;
> > > +               delta_truesize += nskb->truesize;
> > > +
> > > +               skb_push(nskb, -skb_network_offset(nskb) + offset);
> > > +
> > > +               if (!secpath_exists(nskb))
> > > +                       __skb_ext_copy(nskb, skb);
> >
> > Of all the possible extensions, why is this only relevant to secpath?
>
> I wrote this before we had these extensions and adjusted it
> when the extensions where introduced to make it compile again.
> I think we can just copy the extensions unconditionally.
>
> >
> > More in general, this function open codes a variety of skb fields that
> > carry over from skb to nskb. How did you select this subset of fields?
>
> I tried to find the subset of __copy_skb_header() that is needed to copy.
> Some fields of nskb are still valid, and some (csum related) fields
> should not be copied from skb to nskb.

Duplicating that code is kind of fragile wrt new fields being added to
skbs later (such as the recent skb_ext example).

Perhaps we can split __copy_skb_header further and call the
inner part directly.

Or, at least follow the order of operations exactly and add a comment
that this code was taken verbatim from __copy_skb_header.

> > > +
> > > +               memcpy(nskb->cb, skb->cb, sizeof(skb->cb));
> > > +
> > > +               nskb->tstamp = skb->tstamp;
> > > +               nskb->dev = skb->dev;
> > > +               nskb->queue_mapping = skb->queue_mapping;
> > > +
> > > +               nskb->mac_len = skb->mac_len;
> > > +               nskb->mac_header = skb->mac_header;
> > > +               nskb->transport_header = skb->transport_header;
> > > +               nskb->network_header = skb->network_header;
> > > +               skb_dst_copy(nskb, skb);
> > > +
> > > +               skb_headers_offset_update(nskb, skb_headroom(nskb) - skb_headroom(skb));
> > > +               skb_copy_from_linear_data_offset(skb, -tnl_hlen,
> > > +                                                nskb->data - tnl_hlen,
> > > +                                                offset + tnl_hlen);
> > > +
> > > +               if (skb_needs_linearize(nskb, features) &&
> > > +                   __skb_linearize(nskb))
> > > +                       goto err_linearize;
> > > +
> > > +       } while (list_skb);
> > > +
> > > +       skb->truesize = skb->truesize - delta_truesize;
> > > +       skb->data_len = skb->data_len - delta_len;
> > > +       skb->len = skb->len - delta_len;
> > > +       skb->ip_summed = nskb->ip_summed;
> > > +       skb->csum_level = nskb->csum_level;
> >
> > This changed from the previous version, where nskb inherited ip_summed
> > and csum_level from skb. Why is that?
>
> I had to set ip_summed to CHECKSUM_UNNECESSARY on GRO to
> make sure the noone touches the checksum of the head
> skb. Otherise netfilter etc. tries to touch the csum.
>
> Before chaining I make sure that ip_summed and csum_level is
> the same for all chained skbs and here I restore the original
> value from nskb.

This is safe because the skb_gro_checksum_validate will have validated
already on CHECKSUM_PARTIAL? What happens if there is decap or encap
in the path? We cannot revert to CHECKSUM_PARTIAL after that, I
imagine.

Either way, would you mind briefly documenting the checksum behavior
in the commit message? It's not trivial and I doubt I'll recall the
details in six months.

Really about patch 4: that squashed in a lot of non-trivial scaffolding
from previous patch 'UDP: enable GRO by default'. Does it make sense
to keep that in a separate patch? That should be a noop, which we can
verify. And it makes patch 4 easier to reason about on its own, too.