netdev - Re: [PATCH v2 net-next 4/7] ipv6: Plumb support for nexthop object in a fib6

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Date:   Wed, 5 Jun 2019 00:39:05 +0000
From:   Martin Lau <kafai@...com>
To:     Wei Wang <weiwan@...gle.com>
CC:     David Ahern <dsahern@...il.com>, David Ahern <dsahern@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "idosch@...lanox.com" <idosch@...lanox.com>,
        "saeedm@...lanox.com" <saeedm@...lanox.com>
Subject: Re: [PATCH v2 net-next 4/7] ipv6: Plumb support for nexthop object in
 a fib6_info

On Tue, Jun 04, 2019 at 02:36:27PM -0700, Wei Wang wrote:
> On Tue, Jun 4, 2019 at 2:13 PM David Ahern <dsahern@...il.com> wrote:
> >
> > On 6/4/19 3:06 PM, Martin Lau wrote:
> > > On Tue, Jun 04, 2019 at 02:17:28PM -0600, David Ahern wrote:
> > >> On 6/3/19 11:29 PM, Martin Lau wrote:
> > >>> On Mon, Jun 03, 2019 at 07:36:06PM -0600, David Ahern wrote:
> > >>>> On 6/3/19 6:58 PM, Martin Lau wrote:
> > >>>>> I have concern on calling ip6_create_rt_rcu() in general which seems
> > >>>>> to trace back to this commit
> > >>>>> dec9b0e295f6 ("net/ipv6: Add rt6_info create function for ip6_pol_route_lookup")
> > >>>>>
> > >>>>> This rt is not tracked in pcpu_rt, rt6_uncached_list or exception bucket.
> > >>>>> In particular, how to react to NETDEV_UNREGISTER/DOWN like
> > >>>>> the rt6_uncached_list_flush_dev() does and calls dev_put()?
> > >>>>>
> > >>>>> The existing callers seem to do dst_release() immediately without
> > >>>>> caching it, but still concerning.
> > >>>>
> > >>>> those are the callers that don't care about the dst_entry, but are
> > >>>> forced to deal with it. Removing the tie between fib lookups an
> > >>>> dst_entry is again the right solution.
> > >>> Great to know that there will be a solution.  It would be great
> > >>> if there is patch (or repo) to show how that may look like on
> > >>> those rt6_lookup() callers.
> > >>
> > >> Not 'will be', 'there is' a solution now. Someone just needs to do the
> > >> conversions and devise the tests for the impacted users.
> > > I don't think everyone will convert to the new nexthop solution
> > > immediately.
> > >
> > > How about ensuring the existing usage stays solid first?
> >
> > Use of nexthop objects has nothing to do with separating fib lookups
> > from dst_entries, but with the addition of fib6_result it Just Works.
> >
> > Wei converted ipv6 to use exception caches instead of adding them to the
> > FIB.
> >
> > I converted ipv6 to use separate data structures for fib entries, added
> > direct fib6 lookup functions and added fib6_result. See the
> > net/core/filter.c.
> >
> > The stage is set for converting users.
> >
> > For example, ip6_nh_lookup_table does not care about the dst entry, only
> > the fib entry. This converts it:
> >
> > static int ip6_nh_lookup_table(struct net *net, struct fib6_config *cfg,
> >                                const struct in6_addr *gw_addr, u32 tbid,
> >                                int flags, struct fib6_result *res)
> > {
> >         struct flowi6 fl6 = {
> >                 .flowi6_oif = cfg->fc_ifindex,
> >                 .daddr = *gw_addr,
> >                 .saddr = cfg->fc_prefsrc,
> >         };
> >         struct fib6_table *table;
> >         struct rt6_info *rt;
> >
> >         table = fib6_get_table(net, tbid);
> >         if (!table)
> >                 return -EINVAL;
> >
> >         if (!ipv6_addr_any(&cfg->fc_prefsrc))
> >                 flags |= RT6_LOOKUP_F_HAS_SADDR;
> >
> >         flags |= RT6_LOOKUP_F_IGNORE_LINKSTATE;
> >
> >         fib6_table_lookup(net, table, cfg->fc_ifindex, fl6, res, flags);
> >         if (res.f6i == net->ipv6.fib6_null_entry)
> >                 return -ENETUNREACH;
> >
> >         fib6_select_path(net, &res, fl6, oif, false, NULL, flags);
> >
> >         return 0;
> > }
> 
> I do agree with Martin that ip6_create_rt_rcu() seems to be dangerous
> as the dst cache created by this func does not get tracked anywhere
> and it is up to the user to not cache it for too long.
IMO, ip6_create_rt_rcu(), which returns untracked rt, was a mistake
and removing it has been overdue.  Tracking down the unregister dev
bug is not easy.

> But I think David, what you are suggesting is:
> instead of trying to convert ip6_create_rt_rcu() to use the pcpu_dst
> logic, completely get rid of the calling to ip6_create_rt_rcu(), and
> directly return f6i in those cases to the caller. Is that correct?
I am fine with either of these two ways to remove ip6_create_rt_rcu().
Further depending on ip6_create_rt_rcu() in this patch even in
ip6_pol_route_lookup() is arguably neither of these two ways...