netdev - Re: [PATCH RFC net-next 10/20] net/ipv6: move expires into rt6

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Message-ID: <20180228225525.sy3lq6yjuujitsbv@kafai-mbp.dhcp.thefacebook.com>
Date:   Wed, 28 Feb 2018 14:56:05 -0800
From:   Martin KaFai Lau <kafai@...com>
To:     David Ahern <dsahern@...il.com>
CC:     Wei Wang <weiwan@...gle.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Ido Schimmel <idosch@...sch.org>, <roopa@...ulusnetworks.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>
Subject: Re: [PATCH RFC net-next 10/20] net/ipv6: move expires into rt6_info

On Wed, Feb 28, 2018 at 03:25:02PM -0700, David Ahern wrote:
> On 2/28/18 12:21 PM, Martin KaFai Lau wrote:
> > On Mon, Feb 26, 2018 at 03:55:14PM -0700, David Ahern wrote:
> >> On 2/26/18 3:28 PM, Wei Wang wrote:
> >>>> @@ -213,11 +234,6 @@ static inline void rt6_set_expires(struct rt6_info *rt, unsigned long expires)
> >>>>
> >>>>  static inline void rt6_update_expires(struct rt6_info *rt0, int timeout)
> >>>>  {
> >>>> -       struct rt6_info *rt;
> >>>> -
> >>>> -       for (rt = rt0; rt && !(rt->rt6i_flags & RTF_EXPIRES); rt = rt->from);
> >>>> -       if (rt && rt != rt0)
> >>>> -               rt0->dst.expires = rt->dst.expires;
> >>>
> >>> I was wondering if we need to retain the above logic. It makes sure
> >>> dst.expires gets synced to its "parent" route. But  it might be hard
> >>> because after your change, we can no longer use rt->from to refer to
> >>> the "parent".
> >>
> >> As I understand it, the FIB entries are cloned into pcpu, uncached and
> >> exception routes. We should never have an rt6_info that ever points back
> >> more than 1 level -- ie., the dst rt6_info points to a from representing
> >> the original FIB entry.
> > Agree on at most 1 level.
> > 
> >>
> >> After my change 'from' will still point to the FIB entry as a fib6_info
> >> which has its own expires.
> >>
> >> When I looked this code I was really confused. At best, the for loop
> >> above sets rt0->dst.expires to some value based on the 'from' but then
> >> the very next line calls dst_set_expires with the passed in timeout value.
> > My understanding is, the rt0 first inherits the expires from its rt0->from.
> > 
> > The following dst_set_expires() set a new timeout if the new timeout
> > is earlier than the existing expires.  I think it is essentially
> > taking a min.
> > 
> > One question, would avoid taking the min cause the rt0 somehow
> > have a longer expires than its parent (or f6i after this series)?
> 
> I believe the current logic expands to:
> 
> static inline void rt6_update_expires(struct rt6_info *rt0, int timeout)
> {
>         if (!(rt0->rt6i_flags & RTF_EXPIRES) && rt0->from)
>                 rt0->dst.expires = rt0->from->dst.expires;
> 
>         dst_set_expires(&rt0->dst, timeout);
I think the RTF_EXPIRES check should also be skipped.  Otherwise,
the already-exist rt6_cache will not get a new expires after
another pmtu exception.

A nit. It seems rt6_update_expires() is only used by
rt6_do_update_pmtu().  It could be more readable
to move these codes to rt6_do_update_pmtu() such that
we know it is a special handling for pmtu case.
(i.e. remove rt6_update_expires()).

>         rt0->rt6i_flags |= RTF_EXPIRES;
> }
> 
> 
> With the fib6_info I can keep that logic with:
> 
> static inline void rt6_update_expires(struct rt6_info *rt0, int timeout)
> {
>         if (!(rt0->rt6i_flags & RTF_EXPIRES) && rt0->from)
>                 rt0->dst.expires = rt0->from->expires;
> 
>         dst_set_expires(&rt0->dst, timeout);
>         rt0->rt6i_flags |= RTF_EXPIRES;
> }
>