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

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Message-ID: <20190604005840.tiful44xo34lpf6d@kafai-mbp.dhcp.thefacebook.com>
Date:   Tue, 4 Jun 2019 00:58:45 +0000
From:   Martin Lau <kafai@...com>
To:     David Ahern <dsahern@...il.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 Mon, Jun 03, 2019 at 05:18:11PM -0600, David Ahern wrote:
> On 6/3/19 5:05 PM, Wei Wang wrote:
> > On Mon, Jun 3, 2019 at 3:35 PM David Ahern <dsahern@...il.com> wrote:
> >>
> >> On 6/3/19 3:58 PM, Wei Wang wrote:
> >>> Hmm... I am still a bit concerned with the ip6_create_rt_rcu() call.
> >>> If we have a blackholed nexthop, the lookup code here always tries to
> >>> create an rt cache entry for every lookup.
> >>> Maybe we could reuse the pcpu cache logic for this? So we only create
> >>> new dst cache on the CPU if there is no cache created before.
> >>
> >> I'll take a look.
> >>
> 
> BTW, I am only updating ip6_pol_route to use pcpu routes for blackhole
> nexthops.
> 
> ip6_pol_route_lookup will continue as is. That function does not use
> pcpu routes and will stay as is.
> 
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.