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

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Date:   Tue, 4 Jun 2019 15:13:38 -0600
From:   David Ahern <dsahern@...il.com>
To:     Martin Lau <kafai@...com>
Cc:     Wei Wang <weiwan@...gle.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 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;
}