From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
To: Steffen Klassert <steffen.klassert@secunet.com>
Cc: David Miller <davem@davemloft.net>,
Paolo Abeni <pabeni@redhat.com>,
Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>,
Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>,
Network Development <netdev@vger.kernel.org>
Subject: Re: [PATCH net-next 3/4] net: Support GRO/GSO fraglist chaining.
Date: Thu, 19 Dec 2019 11:28:34 -0500 [thread overview]
Message-ID: <CA+FuTScKcwyh7rZdDNQsujndrA+ZnYMmtA7Uh7-ji+RM+t6-hQ@mail.gmail.com> (raw)
In-Reply-To: <20191219082246.GS8621@gauss3.secunet.de>
On Thu, Dec 19, 2019 at 3:22 AM Steffen Klassert
<steffen.klassert@secunet.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@secunet.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.
next prev parent reply other threads:[~2019-12-19 16:29 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-18 13:34 [PATCH net-next 0/4] Support fraglist GRO/GSO Steffen Klassert
2019-12-18 13:34 ` [PATCH net-next 1/4] net: Add fraglist GRO/GSO feature flags Steffen Klassert
2019-12-18 16:02 ` Willem de Bruijn
2019-12-18 13:34 ` [PATCH net-next 2/4] net: Add a netdev software feature set that defaults to off Steffen Klassert
2019-12-18 16:02 ` Willem de Bruijn
2019-12-18 13:34 ` [PATCH net-next 3/4] net: Support GRO/GSO fraglist chaining Steffen Klassert
2019-12-18 16:02 ` Willem de Bruijn
2019-12-19 8:22 ` Steffen Klassert
2019-12-19 16:28 ` Willem de Bruijn [this message]
2020-01-13 8:51 ` Steffen Klassert
2020-01-13 16:21 ` Willem de Bruijn
2020-01-15 9:47 ` Steffen Klassert
2020-01-15 15:43 ` Willem de Bruijn
2020-01-20 8:35 ` Steffen Klassert
2020-01-20 16:35 ` Willem de Bruijn
2019-12-18 13:34 ` [PATCH net-next 4/4] udp: Support UDP fraglist GRO/GSO Steffen Klassert
2019-12-18 16:03 ` Willem de Bruijn
2019-12-19 8:26 ` Steffen Klassert
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=CA+FuTScKcwyh7rZdDNQsujndrA+ZnYMmtA7Uh7-ji+RM+t6-hQ@mail.gmail.com \
--to=willemdebruijn.kernel@gmail.com \
--cc=davem@davemloft.net \
--cc=marcelo.leitner@gmail.com \
--cc=netdev@vger.kernel.org \
--cc=pabeni@redhat.com \
--cc=steffen.klassert@secunet.com \
--cc=subashab@codeaurora.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).