netdev - Re: [PATCH net-next v3] ipv6: add IFLA_INET6_RA_MTU to expose mtu value in the RA message

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Date:   Fri, 13 Aug 2021 16:07:40 +0800
From:   Rocco Yue <rocco.yue@...iatek.com>
To:     David Ahern <dsahern@...il.com>
CC:     "David S . Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-mediatek@...ts.infradead.org>, <rocco.yue@...il.com>,
        <chao.song@...iatek.com>, <zhuoliang.zhang@...iatek.com>,
        Rocco Yue <rocco.yue@...iatek.com>
Subject: Re: [PATCH net-next v3] ipv6: add IFLA_INET6_RA_MTU to expose mtu value in the RA message

On Wed, 2021-08-11 at 12:05 -0600, David Ahern wrote:
> On 8/11/21 7:56 AM, David Ahern wrote:
>> On 8/10/21 6:33 AM, Rocco Yue wrote:
>>> On Mon, 2021-08-09 at 16:43 -0600, David Ahern wrote:
>>>>
>>>> Since this MTU is getting reported via af_info infrastructure,
>>>> rtmsg_ifinfo should be sufficient.
>>>>
>>>> From there use 'ip monitor' to make sure you are not generating multiple
>>>> notifications; you may only need this on the error path.
>>>
>>> Hi David,
>>>
>>> To avoid generating multiple notifications, I added a separate ramtu notify
>>> function in this patch, and I added RTNLGRP_IPV6_IFINFO nl_mgrp to the ipmonitor.c
>>> to verify this patch was as expected.
>>>
>>> I look at the rtmsg_ifinfo code, it should be appropriate and I will use it and
>>> verify it.
>>>
>>> But there's one thing, I'm sorry I didn't fully understand the meaning of this
>>> sentence "you may only need this on the error path". Honestly, I'm not sure what
>>> the error patch refers to, do you mean "if (mtu < IPV6_MIN_MTU || mtu > skb->dev->mtu)" ?
>>>
>> 
>> looks like nothing under:
>>     if (ndopts.nd_opts_mtu && in6_dev->cnf.accept_ra_mtu) {
>> 
>>     }
>> 
>> is going to send a link notification so you can just replace
>> inet6_iframtu_notify with rtmsg_ifinfo in your proposed change.
>> 
> 
> Taking a deeper dive on the code, you do not need to call rtmsg_ifinfo.
> Instead, the existing:
> 
>         /*
>          *      Send a notify if RA changed managed/otherconf flags or
> timer settings
>          */
>         if (send_ifinfo_notify)
>                 inet6_ifinfo_notify(RTM_NEWLINK, in6_dev);
> 
> is called too early. For one the RA can change the MTU and that is done
> after this notify.
> 
> I think if you moved this down to the out:
> 
> out:
>         /*
>          *      Send a notify if RA changed managed/otherconf flags or
> timer settings
>          */
>         if (send_ifinfo_notify)
>                 inet6_ifinfo_notify(RTM_NEWLINK, in6_dev);
> 
> and then set send_ifinfo_notify when the mtu is *changed* by the RA you
> should be good.

Hi David,

Thanks for your suggestion,
this looks like a better choice without adding a separate notification function,
I will modify it and push the next iteration .

Best Regards,
Rocco