Subject: Re: [PATCH net] bonding: reset the slave's mtu when its be changed

From: Veaceslav Falico <[email-masked]>
Date: Fri, 10 Jan 2014 13:19:32 +0100

On Fri, Jan 10, 2014 at 07:32:51PM +0800, Ding Tianhong wrote:
>All slave should have the same mtu with mastet's, and the bond do it when
>enslave the slave, but the user could change the slave's mtu, it will cause
>the master and slave have different mtu, althrough in AB mode, it does not
>matter if the slave is not the current slave, but in other mode, it is incorrect,
>so reset the slave's mtu like the master set.

Why "net"? It's not a bugfix, it's a feature, and really discussable.

Also, wrt the actual change - why do you think it's incorrect for slaves in
bonding mode other than AB to have different MTU values? I don't see any
reason for it, from the top of the head.

>
>Cc: Jay Vosburgh <[email-masked]>
>Cc: Veaceslav Falico <[email-masked]>
>Signed-off-by: Ding Tianhong <[email-masked]>
>---
> drivers/net/bonding/bond_main.c | 21 ++++++++++-----------
> 1 file changed, 10 insertions(+), 11 deletions(-)
>
>diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
>index 398e299..e7b5bcf 100644
>--- a/drivers/net/bonding/bond_main.c
>+++ b/drivers/net/bonding/bond_main.c
>@@ -2882,18 +2882,17 @@ static int bond_slave_netdev_event(unsigned long event,
> 		 */
> 		break;
> 	case NETDEV_CHANGEMTU:
>-		/*
>-		 * TODO: Should slaves be allowed to
>-		 * independently alter their MTU?  For
>-		 * an active-backup bond, slaves need
>-		 * not be the same type of device, so
>-		 * MTUs may vary.  For other modes,
>-		 * slaves arguably should have the
>-		 * same MTUs. To do this, we'd need to
>-		 * take over the slave's change_mtu
>-		 * function for the duration of their
>-		 * servitude.
>+		/* All slave should have the same mtu
>+		 * as master.
> 		 */
>+		if (slave->dev->mtu != bond->dev->mtu) {

If we've got the event then it means it was changed to something different.
No need to verify.

>+			int res;
>+			slave->original_mtu = slave->dev->mtu;

If we're refusing to apply the *new* mtu, then why should we save it as the
original? The original_mtu is the mtu that the slave had before it was
enslaved.

>+			res = dev_set_mtu(slave->dev, bond->dev->mtu);
>+			if (res)
>+				pr_debug("Error %d calling dev_set_mtu for slave %s\n",
>+					 res, slave->dev->name);
>+		}

Also, bonding should be vocal about changing forcibly the mtu - otherwise
we'd end up with silently dropping the changes:

ifconfig eth0 mtu 9000
echo $?
-> 0
ifconfig eth0
MTU: 1500

or something like that, it will pass it up, refusing changes:

diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
index e06c445..0b36045 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -2846,19 +2846,8 @@ static int bond_slave_netdev_event(unsigned long event,
  		 */
  		break;
  	case NETDEV_CHANGEMTU:
-		/*
-		 * TODO: Should slaves be allowed to
-		 * independently alter their MTU?  For
-		 * an active-backup bond, slaves need
-		 * not be the same type of device, so
-		 * MTUs may vary.  For other modes,
-		 * slaves arguably should have the
-		 * same MTUs. To do this, we'd need to
-		 * take over the slave's change_mtu
-		 * function for the duration of their
-		 * servitude.
-		 */
-		break;
+		/* don't permit slaves to change their MTU */
+		return NOTIFY_BAD;
  	case NETDEV_CHANGENAME:
  		/*
  		 * TODO: handle changing the primary's name

> 		break;
> 	case NETDEV_CHANGENAME:
> 		/*
>--
>1.8.0