Message-ID: <52661fed-f521-4cdc-b9e1-b4f3fa292e78@openvpn.net>
Date: Thu, 21 Nov 2024 22:23:36 +0100
From: Antonio Quartulli <antonio@...nvpn.net>
To: Sergey Ryazanov <ryazanov.s.a@...il.com>,
 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 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 :-)
The switch is much more elegant and straightforward.

Do you agree this is getting more into a bike shed coloring discussion? :-D

Since there is not much gain in changing approach, I think it is better 
if the maintainer picks a style that he finds more suitable (or simply 
likes more). no?

> 
>>> And back to the hashtable(s) size for the MP mode. 8k-bins table looks a
>>> good choice for a normal server with 1-2Gb uplink serving up to 1k
>>> connections. But it sill unclear, how this choice can affect 
>>> installations
>>> with a bigger number of connections? Or is this module applicable for
>>> embedded solutions? E.g. running a couple of VPN servers on a home 
>>> router
>>> with a few actual connections looks like a waste of RAM. I was about to
>>> suggest to use rhashtable due to its dynamic sizing feature, but the 
>>> module
>>> needs three tables. Any better idea?
>>
>> For this initial implementation I think it's fine. Sure, converting to
>> rhashtable (or some other type of dynamically-sized hashtable, if
>> rhashtable doesn't fit) in the future would make sense. But I don't
>> think it's necessary to get the patches into net-next.

Agreed. It's in the pipe (along with other features that I have already 
implemented), but it will come later.

Regards,

> 
> Make sense. Thanks for sharing these thoughts.
> 
> -- 
> Sergey

-- 
Antonio Quartulli
OpenVPN Inc.

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