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

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Message-ID: <040a1551-2a9f-18d0-9987-f196bb429c1b@blackwall.org>
Date:   Wed, 25 May 2022 11:38:37 +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 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.

>> 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