Message-ID: <dc1b37b2-389a-466d-8559-14c496cc9583@gmail.com>
Date: Thu, 21 Nov 2024 01:22:44 +0200
From: Sergey Ryazanov <ryazanov.s.a@...il.com>
To: Sabrina Dubroca <sd@...asysnail.net>
Cc: Antonio Quartulli <antonio@...nvpn.net>,
 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 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.:

next_run = ovpn->mode == OVPN_MODE_P2P ?
            ovpn_peer_keepalive_work_p2p(...) :
            ovpn_peer_keepalive_work_mp(...);

>> 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.

Make sense. Thanks for sharing these thoughts.

--
Sergey

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