linux-kernel - Re: [PATCH net-next v11 06/23] ovpn: introduce the ovpn

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Message-ID: <a1d70ccf-9731-4ec1-b4c3-606ee7073c18@gmail.com>
Date: Sat, 23 Nov 2024 23:05:56 +0200
From: Sergey Ryazanov <ryazanov.s.a@...il.com>
To: Antonio Quartulli <antonio@...nvpn.net>,
 Sabrina Dubroca <sd@...asysnail.net>
Cc: Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Donald Hunter <donald.hunter@...il.com>,
 Shuah Khan <shuah@...nel.org>, Andrew Lunn <andrew@...n.ch>,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-kselftest@...r.kernel.org
Subject: Re: [PATCH net-next v11 06/23] ovpn: introduce the ovpn_peer object

On 21.11.2024 23:23, Antonio Quartulli wrote:
> On 21/11/2024 00:22, Sergey Ryazanov wrote:
>> On 13.11.2024 12:03, Sabrina Dubroca wrote:
>>> 2024-11-13, 03:37:13 +0200, Sergey Ryazanov wrote:
>>>> On 12.11.2024 19:31, Sabrina Dubroca wrote:
>>>>> 2024-11-10, 15:38:27 +0200, Sergey Ryazanov wrote:
>>>>>> On 29.10.2024 12:47, Antonio Quartulli wrote:
>>>>>>> An ovpn_peer object holds the whole status of a remote peer
>>>>>>> (regardless whether it is a server or a client).
>>>>>>>
>>>>>>> This includes status for crypto, tx/rx buffers, napi, etc.
>>>>>>>
>>>>>>> Only support for one peer is introduced (P2P mode).
>>>>>>> Multi peer support is introduced with a later patch.
>>>>>>
>>>>>> Reviewing the peer creation/destroying code I came to a generic 
>>>>>> question.
>>>>>> Did you consider keeping a single P2P peer in the peers table as 
>>>>>> well?
>>>>>>
>>>>>> Looks like such approach can greatly simply the code by dropping 
>>>>>> all these
>>>>>> 'switch (ovpn->mode)' checks and implementing a unified peer 
>>>>>> management. The
>>>>>> 'peer' field in the main private data structure can be kept to 
>>>>>> accelerate
>>>>>> lookups, still using peers table for management tasks like 
>>>>>> removing all the
>>>>>> peers on the interface teardown.
>>>>>
>>>>> It would save a few 'switch(mode)', but force every client to allocate
>>>>> the hashtable for no reason at all. That tradeoff doesn't look very
>>>>> beneficial to me, the P2P-specific code is really simple. And if you
>>>>> keep ovpn->peer to make lookups faster, you're not removing that many
>>>>> 'switch(mode)'.
>>>>
>>>> Looking at the done review, I can retrospectively conclude that I 
>>>> personally
>>>> do not like short 'switch' statements and special handlers :)
>>>>
>>>> Seriously, this module has a highest density of switches per KLOC 
>>>> from what
>>>> I have seen before and a major part of it dedicated to handle the 
>>>> special
>>>> case of P2P connection.
>>>
>>> I think it's fine. Either way there will be two implementations of
>>> whatever mode-dependent operation needs to be done. switch doesn't
>>> make it more complex than an ops structure.
>>>
>>> If you're reading the current version and find ovpn_peer_add, you see
>>> directly that it'll do either ovpn_peer_add_mp or
>>> ovpn_peer_add_p2p. With an ops structure, you'd have a call to
>>> ovpn->ops->peer_add, and you'd have to look up all possible ops
>>> structures to know that it can be either ovpn_peer_add_mp or
>>> ovpn_peer_add_p2p. If there's an undefined number of implementations
>>> living in different modules (like net_device_ops, or L4 protocols),
>>> you don't have a choice.
>>>
>>> xfrm went the opposite way to what you're proposing a few years ago
>>> (see commit 0c620e97b349 ("xfrm: remove output indirection from
>>> xfrm_mode") and others), and it made the code simpler.
>>
>> I checked this. Florian did a nice rework. And the way of 
>> implementation looks reasonable since there are more than two 
>> encapsulation modes and handling is more complex than just selecting a 
>> function to call.
>>
>> What I don't like about switches, that it requires extra lines of code 
>> and pushes an author to introduce a default case with error handling. 
>> It was mentioned that the module unlikely going to support more than 
>> two modes. In this context shall we consider ternary operator usage. 
>> E.g.:
> 
> the default case can actually be dropped. That way we can have the 
> compiler warn when one of the enum values is not handled in the switch 
> (should there be a new one at some point).
> However, the default is just a sanity check against future code changes 
> which may introduce a bug.
> 
>>
>> next_run = ovpn->mode == OVPN_MODE_P2P ?
>>             ovpn_peer_keepalive_work_p2p(...) :
>>             ovpn_peer_keepalive_work_mp(...);
> 
> I find this ugly to read :-)

Yeah. Doesn't look pretty as well.

Just to conclude the discussion. Considering what we discussed here and 
the Sabrina's point regarding the trampoline penalty for indirect 
invocation, we do not have a better solution for now other than using 
switches everywhere.

--
Sergey