netdev - Re: [PATCH net] vlan:make mtu of vlan equal to physical dev

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Message-ID: <4E9E3B3E.2070902@candelatech.com>
Date:	Tue, 18 Oct 2011 19:51:42 -0700
From:	Ben Greear <greearb@...delatech.com>
To:	WeipingPan <panweiping3@...il.com>
CC:	herbert@...dor.hengli.com.au,
	"open list:NETWORKING [GENERAL]" <netdev@...r.kernel.org>
Subject: Re: [PATCH net] vlan:make mtu of vlan equal to physical dev

On 10/18/2011 07:06 PM, WeipingPan wrote:
> Hi, Herbert,
>
> What do you think of this patch ?
>
> thanks
> Weiping Pan
> On 10/08/2011 06:12 PM, Weiping Pan wrote:
>> Default mtu of vlan device is the same with mtu of physical device,
>> for example 1500, but when change physics mtu to 1600,
>> VLAN device's mtu is still 1500.
>> Certainly, you can change vlan device's mtu to 1600 manually,
>> but I think when you change physics device's mtu, VLAN's mtu should be changed
>> automatically instead of by manually.

I don't like the idea.  It's perfectly valid to have the physical dev with 9000 MTU
and have vlans with 1500 MTU.

>> Steps to Reproduce:
>> 1.vconfig add eth4 3
>> 2.ifconfig eth4 mtu 1600
>> 3.check mtu on eth4.3
>>
>> And what's worse is that if you decrease mtu of pyhsical device,
>> and when you want to increase it, the mtu of vlan device won't be changed.

That *might* be worth fixing, but even so, some NICs might handle that
just fine, so my opinion is that this should not change either.

Thanks,
Ben

>>
>> Steps to Reproduce:
>> 1.vconfig add eth4 3
>> 2.ifconfig eth4 mtu 100
>> 3.ifconfig eth4 mtu 1500
>> 4.the mtu of eth4.3 is still 100
>>
>> This bug is reported by Liang Zheng(lzheng@...hat.com).
>>
>> Signed-off-by: Weiping Pan<panweiping3@...il.com>
>> ---
>> net/8021q/vlan.c | 3 ---
>> 1 files changed, 0 insertions(+), 3 deletions(-)
>>
>> diff --git a/net/8021q/vlan.c b/net/8021q/vlan.c
>> index 8970ba1..f6072b4 100644
>> --- a/net/8021q/vlan.c
>> +++ b/net/8021q/vlan.c
>> @@ -417,9 +417,6 @@ static int vlan_device_event(struct notifier_block *unused, unsigned long event,
>> if (!vlandev)
>> continue;
>>
>> - if (vlandev->mtu<= dev->mtu)
>> - continue;
>> -
>> dev_set_mtu(vlandev, dev->mtu);
>> }
>> break;
>
> --
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-- 
Ben Greear <greearb@...delatech.com>
Candela Technologies Inc  http://www.candelatech.com
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