linux-kernel - Re: [PATCH V3 net-next 1/4] net: bridge: add fdb flag to extent locked port feature

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Message-ID: <4bf1c80d-0f18-f444-3005-59a45797bcfd@blackwall.org>
Date:   Wed, 25 May 2022 13:18:49 +0300
From:   Nikolay Aleksandrov <razor@...ckwall.org>
To:     Hans Schultz <schultz.hans@...il.com>, davem@...emloft.net,
        kuba@...nel.org
Cc:     netdev@...r.kernel.org, Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>, Jiri Pirko <jiri@...nulli.us>,
        Ivan Vecera <ivecera@...hat.com>,
        Roopa Prabhu <roopa@...dia.com>, Shuah Khan <shuah@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Ido Schimmel <idosch@...dia.com>, linux-kernel@...r.kernel.org,
        bridge@...ts.linux-foundation.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH V3 net-next 1/4] net: bridge: add fdb flag to extent
 locked port feature

On 25/05/2022 12:11, Hans Schultz wrote:
> On ons, maj 25, 2022 at 11:38, Nikolay Aleksandrov <razor@...ckwall.org> wrote:
>> On 25/05/2022 11:34, Hans Schultz wrote:
>>> On ons, maj 25, 2022 at 11:06, Nikolay Aleksandrov <razor@...ckwall.org> wrote:
>>>> On 24/05/2022 19:21, Hans Schultz wrote:
>>>>>>
>>>>>> Hi Hans,
>>>>>> So this approach has a fundamental problem, f->dst is changed without any synchronization
>>>>>> you cannot rely on it and thus you cannot account for these entries properly. We must be very
>>>>>> careful if we try to add any new synchronization not to affect performance as well.
>>>>>> More below...
>>>>>>
>>>>>>> @@ -319,6 +326,9 @@ static void fdb_delete(struct net_bridge *br, struct net_bridge_fdb_entry *f,
>>>>>>>  	if (test_bit(BR_FDB_STATIC, &f->flags))
>>>>>>>  		fdb_del_hw_addr(br, f->key.addr.addr);
>>>>>>>  
>>>>>>> +	if (test_bit(BR_FDB_ENTRY_LOCKED, &f->flags) && !test_bit(BR_FDB_OFFLOADED, &f->flags))
>>>>>>> +		atomic_dec(&f->dst->locked_entry_cnt);
>>>>>>
>>>>>> Sorry but you cannot do this for multiple reasons:
>>>>>>  - f->dst can be NULL
>>>>>>  - f->dst changes without any synchronization
>>>>>>  - there is no synchronization between fdb's flags and its ->dst
>>>>>>
>>>>>> Cheers,
>>>>>>  Nik
>>>>>
>>>>> Hi Nik,
>>>>>
>>>>> if a port is decoupled from the bridge, the locked entries would of
>>>>> course be invalid, so maybe if adding and removing a port is accounted
>>>>> for wrt locked entries and the count of locked entries, would that not
>>>>> work?
>>>>>
>>>>> Best,
>>>>> Hans
>>>>
>>>> Hi Hans,
>>>> Unfortunately you need the correct amount of locked entries per-port if you want
>>>> to limit their number per-port, instead of globally. So you need a
>>>> consistent
>>>
>>> Hi Nik,
>>> the used dst is a port structure, so it is per-port and not globally.
>>>
>>> Best,
>>> Hans
>>>
>>
>> Yeah, I know. :) That's why I wrote it, if the limit is not a feature requirement I'd suggest
>> dropping it altogether, it can be enforced externally (e.g. from user-space) if needed.
>>
>> By the way just fyi net-next is closed right now due to merge window. And one more
>> thing please include a short log of changes between versions when you send a new one.
>> I had to go look for v2 to find out what changed.
>>
> 
> Okay, I will drop the limit in the bridge module, which is an easy thing
> to do. :) (It is mostly there to ensure against DOS attacks if someone
> bombards a locked port with random mac addresses.)
> I have a similar limitation in the driver, which should then probably be
> dropped too?
> 

That is up to you/driver, I'd try looking for similar problems in other switch drivers
and check how those were handled. There are people in the CC above that can
directly answer that. :)

> The mayor difference between v2 and v3 is in the mv88e6xxx driver, where
> I now keep an inventory of locked ATU entries and remove them based on a
> timer (mv88e6xxx_switchcore.c).
> 

ack

> I guess the mentioned log should be in the cover letter part?
> 

Yep, usually a short mention of what changed to make it easier for reviewers.
Some people also add the patch-specific changes to each patch under the ---
so they're not included in the log, but I'm fine either way as long as I don't
have to go digging up the old versions.

> 
>>>> fdb view with all its attributes when changing its dst in this case, which would
>>>> require new locking because you have multiple dependent struct fields and it will
>>>> kill roaming/learning scalability. I don't think this use case is worth the complexity it
>>>> will bring, so I'd suggest an alternative - you can monitor the number of locked entries
>>>> per-port from a user-space agent and disable port learning or some similar solution that
>>>> doesn't require any complex kernel changes. Is the limit a requirement to add the feature?
>>>>
>>>> I have an idea how to do it and to minimize the performance hit if it really is needed
>>>> but it'll add a lot of complexity which I'd like to avoid if possible.
>>>>
>>>> Cheers,
>>>>  Nik