netdev - Re: [PATCHv2 net] ipv6/route: should not update neigh confirm time during PMTU update

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Message-Id: <20191203.115818.1902434596879929857.davem@davemloft.net>
Date:   Tue, 03 Dec 2019 11:58:18 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     liuhangbin@...il.com
Cc:     netdev@...r.kernel.org, ja@....bg, marcelo.leitner@...il.com,
        dsahern@...il.com, edumazet@...gle.com
Subject: Re: [PATCHv2 net] ipv6/route: should not update neigh confirm time
 during PMTU update

From: Hangbin Liu <liuhangbin@...il.com>
Date: Tue, 3 Dec 2019 18:25:35 +0800

> 
> Hi David,
> On Tue, Dec 03, 2019 at 06:15:36PM +0800, Hangbin Liu wrote:
>> On Mon, Dec 02, 2019 at 06:47:04PM -0800, David Miller wrote:
>> > From: Hangbin Liu <liuhangbin@...il.com>
>> > Date: Tue,  3 Dec 2019 10:11:37 +0800
>> > 
>> > > Fix it by removing the dst_confirm_neigh() in __ip6_rt_update_pmtu() as
>> > > there is no two-way communication during PMTU update.
>> > > 
>> > > v2: remove dst_confirm_neigh directly as David Miller pointed out.
>> > 
>> > That's not what I said.
>> > 
>> > I said that this interface is designed for situations where the neigh
>> > update is appropriate, and that's what happens for most callers _except_
>> > these tunnel cases.
>> > 
>> > The tunnel use is the exception and invoking the interface
>> > inappropriately.
>> > 
>> > It is important to keep the neigh reachability fresh for TCP flows so
>> > you cannot remove this dst_confirm_neigh() call.
> 
> I have one question here. Since we have the .confirm_neigh fuction in
> struct dst_ops. How about do a dst->ops->confirm_neigh() separately after
> dst->ops->update_pmtu()? Why should we mix the confirm_neigh() in
> update_pmtu(), like ip6_rt_update_pmtu()?

Two indirect calls which have high cost due to spectre mitigation?