Re: [PATCH net-next 01/11] ipv6: optimise ipcm6 cookie init

[Pavel Begunkov]

On 4/28/22 15:04, Paolo Abeni wrote:
> On Thu, 2022-04-28 at 11:56 +0100, Pavel Begunkov wrote:
> > Users of ipcm6_init() have a somewhat complex post initialisation
> > of ->dontfrag and ->tclass. Not only it adds additional overhead,
> > but also complicates the code.
> >
> > First, replace ipcm6_init() with ipcm6_init_sk(). As it might be not an
> > equivalent change, let's first look at ->dontfrag. The logic was to set
> > it from cmsg if specified and otherwise fallback to np->dontfrag. Now
> > it's initialising to np->dontfrag in the beginning and then potentially
> > overriding with cmsg, which is absolutely the same behaviour.
> >
> > It's a bit more complex with ->tclass as ip6_datagram_send_ctl() might
> > set it to -1, which is a default and not valid value. The solution
> > here is to skip -1's specified in cmsg, so it'll be left with the socket
> > default value getting us to the old behaviour.
> >
> > Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
> > ---
> >   include/net/ipv6.h    | 9 ---------
> >   net/ipv6/datagram.c   | 4 ++--
> >   net/ipv6/ip6_output.c | 2 --
> >   net/ipv6/raw.c        | 8 +-------
> >   net/ipv6/udp.c        | 7 +------
> >   net/l2tp/l2tp_ip6.c   | 8 +-------
> >   6 files changed, 5 insertions(+), 33 deletions(-)
> >
> > diff --git a/include/net/ipv6.h b/include/net/ipv6.h
> > index 213612f1680c..30a3447e34b4 100644
> > --- a/include/net/ipv6.h
> > +++ b/include/net/ipv6.h
> > @@ -352,15 +352,6 @@ struct ipcm6_cookie {
> >   	struct ipv6_txoptions *opt;
> >   };
> >   
> > -static inline void ipcm6_init(struct ipcm6_cookie *ipc6)
> > -{
> > -	*ipc6 = (struct ipcm6_cookie) {
> > -		.hlimit = -1,
> > -		.tclass = -1,
> > -		.dontfrag = -1,
> > -	};
> > -}
> > -
> >   static inline void ipcm6_init_sk(struct ipcm6_cookie *ipc6,
> >   				 const struct ipv6_pinfo *np)
> >   {
> > diff --git a/net/ipv6/datagram.c b/net/ipv6/datagram.c
> > index 206f66310a88..1b334bc855ae 100644
> > --- a/net/ipv6/datagram.c
> > +++ b/net/ipv6/datagram.c
> > @@ -1003,9 +1003,9 @@ int ip6_datagram_send_ctl(struct net *net, struct sock *sk,
> >   			if (tc < -1 || tc > 0xff)
> >   				goto exit_f;
> >   
> > +			if (tc != -1)
> > +				ipc6->tclass = tc;
> >   			err = 0;
> > -			ipc6->tclass = tc;
> > -
> >   			break;
> >   		    }
>
> It looks like the above causes a behavioral change: before this patch
> cmsg took precedence on socket status, after this patch looks like it's
> the opposide.
>
> Am I missing something?

before:

ipc6.tclass = -1;
if (cmsg)
	ip6_datagram_send_ctl(&ipc6);
if (ipc6.tclass < 0)
	ipc6.tclass = np->tclass;

after:

ipc6.tclass = np->tclass; // ipcm6_init_sk()
if (cmsg)
	ip6_datagram_send_ctl(&ipc6);


Both should prioritise cmsg. The only catch is when tclass is
specified in cmsg but it's -1. The old version would assign
np->tclass in the end, the new one does the same but with
this added "if" in ip6_datagram_send_ctl() in the chunk
you quoted. Unless I missed something as well.

--
Pavel Begunkov