netdev - Re: [PATCH net-next v2 1/1] net: neigh: persist proxy config across link flaps

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Date:   Thu, 15 Dec 2022 12:08:34 -0800
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     David Decotigny <decot@...gle.com>
Cc:     David Decotigny <decot+git@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Nikolay Aleksandrov <razor@...ckwall.org>,
        David Ahern <dsahern@...nel.org>,
        "Denis V. Lunev" <den@...nvz.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Chen Zhongjin <chenzhongjin@...wei.com>,
        Yuwei Wang <wangyuweihx@...il.com>,
        Alexander Mikhalitsyn <alexander.mikhalitsyn@...tuozzo.com>,
        Thomas Zeitlhofer <thomas.zeitlhofer+lkml@...it.at>
Subject: Re: [PATCH net-next v2 1/1] net: neigh: persist proxy config across
 link flaps

On Thu, Dec 15, 2022 at 9:29 AM David Decotigny <decot@...gle.com> wrote:
>
>
> (comments inline below)
>
>
> On Thu, Dec 15, 2022 at 8:24 AM Alexander H Duyck <alexander.duyck@...il.com> wrote:
>>
>> On Wed, 2022-12-14 at 15:20 -0800, David Decotigny wrote:
>> > From: David Decotigny <ddecotig@...gle.com>
>> >
>> > Without this patch, the 'ip neigh add proxy' config is lost when the
>> > cable or peer disappear, ie. when the link goes down while staying
>> > admin up. When the link comes back, the config is never recovered.
>> >
>> > This patch makes sure that such an nd proxy config survives a switch
>> > or cable issue.
>> >
>> > Signed-off-by: David Decotigny <ddecotig@...gle.com>
>> >
>> >
>> > ---
>> > v1: initial revision
>> > v2: same as v1, except rebased on top of latest net-next, and includes "net-next" in the description
>> >
>> >  net/core/neighbour.c | 5 ++++-
>> >  1 file changed, 4 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/net/core/neighbour.c b/net/core/neighbour.c
>> > index f00a79fc301b..f4b65bbbdc32 100644
>> > --- a/net/core/neighbour.c
>> > +++ b/net/core/neighbour.c
>> > @@ -426,7 +426,10 @@ static int __neigh_ifdown(struct neigh_table *tbl, struct net_device *dev,
>> >  {
>> >       write_lock_bh(&tbl->lock);
>> >       neigh_flush_dev(tbl, dev, skip_perm);
>> > -     pneigh_ifdown_and_unlock(tbl, dev);
>> > +     if (skip_perm)
>> > +             write_unlock_bh(&tbl->lock);
>> > +     else
>> > +             pneigh_ifdown_and_unlock(tbl, dev);
>> >       pneigh_queue_purge(&tbl->proxy_queue, dev ? dev_net(dev) : NULL,
>> >                          tbl->family);
>> >       if (skb_queue_empty_lockless(&tbl->proxy_queue))
>>
>> This seems like an agressive approach since it applies to all entries
>> in the table, not just the permenant ones like occurs in
>> neigh_flush_dev.
>>
>> I don't have much experience in this area of the code but it seems like
>> you would specifically be wanting to keep only the permanant entries.
>> Would it make sense ot look at rearranging pneigh_ifdown_and_unlock so
>> that the code functioned more like neigh_flush_dev where it only
>> skipped the permanant entries when skip_perm was set?
>>
>
> The reason I am proposing this patch like it is is because these "proxy" entries appear to be a configuration attribute (similar to ip routes, coming from the sysadmin config), and not cached data (like ip neigh "normal" entries essentially coming from the outside). So I view them as fundamentally different kinds of objects [1], which they actually are in the code. And they are also updated from a vastly different context (sysadmin vs traffic). IMHO, it would seem natural that these proxy attributes (considered config attributes) would survive link flaps, whereas normal ip neigh cached entries without NUD_PERMANENT should not. And neither should survive admin down, the same way ip route does not survive admin down. This is what this patch proposes.
>
> Honoring NUD_PERMANENT (I assume that's what you are alluding to) would also work, and (with current iproute2 implementation [2]) would lead to the same result. But please consider the above. If really honoring NUD_PERMANENT is the required approach here, I am happy to revisit this patch. Please let me know.

Yeah, I was referring to basically just limiting your changes to honor
NUD_PERMANANT. Looking at pneigh_ifdown_and_unlock and comparing it to
neigh_flush_dev it seems like it would make sense to just add the
skip_perm argument there and then add the same logic at the start of
the loop to eliminate the items you aren't going to flush/free. That
way we aren't keeping around any more entries than those specifically
that are supposed to be permanent.