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

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

[Weiping Pan]

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.

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.

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@redhat.com).

Signed-off-by: Weiping Pan <panweiping3@gmail.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;
-- 
1.7.4.4

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

[David Miller]

From: Weiping Pan <panweiping3@gmail.com>
Date: Sat,  8 Oct 2011 18:12:15 +0800

[ Herbert, Weiping is suggesting to remove the "vlandev->mtu <= dev->mtu"
  test in your patch below. ]

> 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.
> 
> 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.
> 
> 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@redhat.com).
> 
> Signed-off-by: Weiping Pan <panweiping3@gmail.com>

But now this setting is unconditional, and thus value user made
settings will be ignored.

The code that is there now is just a safety measure, and the
test appears to very much be intentional:

--------------------
commit 2e477c9bd2bb6a1606e498adb53ba913378ecdf2
Author: Herbert Xu <herbert@gondor.apana.org.au>
Date:   Mon Jul 20 07:35:37 2009 -0700

    vlan: Propagate physical MTU changes
    
    When the physical MTU changes we want to ensure that all existing
    VLAN device MTUs do not exceed the new underlying MTU.  This patch
    adds that propagation.
    
    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
    Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/8021q/vlan.c b/net/8021q/vlan.c
index fe64908..6d37b7e 100644
--- a/net/8021q/vlan.c
+++ b/net/8021q/vlan.c
@@ -468,6 +468,19 @@ static int vlan_device_event(struct notifier_block *unused, unsigned long event,
 		}
 		break;
 
+	case NETDEV_CHANGEMTU:
+		for (i = 0; i < VLAN_GROUP_ARRAY_LEN; i++) {
+			vlandev = vlan_group_get_device(grp, i);
+			if (!vlandev)
+				continue;
+
+			if (vlandev->mtu <= dev->mtu)
+				continue;
+
+			dev_set_mtu(vlandev, dev->mtu);
+		}
+		break;
+
 	case NETDEV_FEAT_CHANGE:
 		/* Propagate device features to underlying device */
 		for (i = 0; i < VLAN_GROUP_ARRAY_LEN; i++) {

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

[WeipingPan]

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.
>
> 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.
>
> 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@redhat.com).
>
> Signed-off-by: Weiping Pan<panweiping3@gmail.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;

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

[Ben Greear]

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@redhat.com).
>>
>> Signed-off-by: Weiping Pan<panweiping3@gmail.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;
>
> --
> To unsubscribe from this list: send the line "unsubscribe netdev" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at http://vger.kernel.org/majordomo-info.html


-- 
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc  http://www.candelatech.com

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

[Herbert Xu]

On Wed, Oct 19, 2011 at 10:06:48AM +0800, WeipingPan wrote:
>
> What do you think of this patch ?

I think the current behaviour is fine as it is.

Cheers,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt