Message-ID: <19256280-594a-7399-9119-ded712c4dbb8@oracle.com>
Date:   Mon, 8 Apr 2019 14:48:57 -0700
From:   si-wei liu <si-wei.liu@...cle.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     sridhar.samudrala@...el.com, stephen@...workplumber.org,
        davem@...emloft.net, kubakici@...pl, alexander.duyck@...il.com,
        jiri@...nulli.us, netdev@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, liran.alon@...cle.com,
        boris.ostrovsky@...cle.com, vijay.balakrishna@...cle.com
Subject: Re: [PATCH net v7] failover: allow name change on IFF_UP slave
 interfaces



On 4/8/2019 2:00 PM, Michael S. Tsirkin wrote:
> On Mon, Apr 08, 2019 at 04:24:45PM -0400, Si-Wei Liu wrote:
>> When a netdev appears through hot plug then gets enslaved by a failover
>> master that is already up and running, the slave will be opened
>> right away after getting enslaved. Today there's a race that userspace
>> (udev) may fail to rename the slave if the kernel (net_failover)
>> opens the slave earlier than when the userspace rename happens.
>> Unlike bond or team, the primary slave of failover can't be renamed by
>> userspace ahead of time, since the kernel initiated auto-enslavement is
>> unable to, or rather, is never meant to be synchronized with the rename
>> request from userspace.
>>
>> As the failover slave interfaces are not designed to be operated
>> directly by userspace apps: IP configuration, filter rules with
>> regard to network traffic passing and etc., should all be done on master
>> interface. In general, userspace apps only care about the
>> name of master interface, while slave names are less important as long
>> as admin users can see reliable names that may carry
>> other information describing the netdev. For e.g., they can infer that
>> "ens3nsby" is a standby slave of "ens3", while for a
>> name like "eth0" they can't tell which master it belongs to.
>>
>> Historically the name of IFF_UP interface can't be changed because
>> there might be admin script or management software that is already
>> relying on such behavior and assumes that the slave name can't be
>> changed once UP. But failover is special: with the in-kernel
>> auto-enslavement mechanism, the userspace expectation for device
>> enumeration and bring-up order is already broken. Previously initramfs
>> and various userspace config tools were modified to bypass failover
>> slaves because of auto-enslavement and duplicate MAC address. Similarly,
>> in case that users care about seeing reliable slave name, the new type
>> of failover slaves needs to be taken care of specifically in userspace
>> anyway.
>>
>> It's less risky to lift up the rename restriction on failover slave
>> which is already UP. Although it's possible this change may potentially
>> break userspace component (most likely configuration scripts or
>> management software) that assumes slave name can't be changed while
>> UP, it's relatively a limited and controllable set among all userspace
>> components, which can be fixed specifically to listen for the rename
>> and/or link down/up events on failover slaves.
> This was correct with v6. With v7 link down/up events are no longer
> emitted.
Oops, missed to fix the commit message. Will send out another rev with 
it corrected.

>
>> Userspace component
>> interacting with slaves is expected to be changed to operate on failover
>> master interface instead, as the failover slave is dynamic in nature
>> which may come and go at any point.  The goal is to make the role of
>> failover slaves less relevant, and userspace components should only
>> deal with failover master in the long run.
>>
>> Fixes: 30c8bd5aa8b2 ("net: Introduce generic failover module")
>> Signed-off-by: Si-Wei Liu <si-wei.liu@...cle.com>
>> Reviewed-by: Liran Alon <liran.alon@...cle.com>
>>
>> --
>> v1 -> v2:
>> - Drop configurable module parameter (Sridhar)
>>
>> v2 -> v3:
>> - Drop additional IFF_SLAVE_RENAME_OK flag (Sridhar)
>> - Send down and up events around rename (Michael S. Tsirkin)
>>
>> v3 -> v4:
>> - Simplify notification to be sent (Stephen Hemminger)
>>
>> v4 -> v5:
>> - Sync up code with latest net-next (Sridhar)
>> - Use proper structure initialization (Stephen, Jiri)
>>
>> v5 -> v6:
>> - Make the property of live name change a generic flag (Stephen)
>>
>> v6 -> v7:
>> - Remove NETDEV_CHANGE notification that is deemed unnecessary
>>    (Stephen)
>> ---
> I preferred v6 better. If we emit NETDEV_CHANGE then
> userspace that expects that name never changes for up
> interfaces will work since it will think state changed.
Can you please settle it down with Stephen for what should be doing? If 
there indeed exists such userspace app it may have expected the link 
state snapshot in the netlink route message is DOWN while detecting the 
the name has changed, hence in this case NETDEV_CHANGE notifier doesn't 
help either as it just reflects the ending state. However, it looks like 
most apps just don't check link state once it attempts to detect rename. 
Even if very few performs the check, nothing is being prohibited and 
those scenarios are even less relevant to failover users.

-Siwei

>
>>   include/linux/netdevice.h |  3 +++
>>   net/core/dev.c            | 16 +++++++++++++++-
>>   net/core/failover.c       |  6 +++---
>>   3 files changed, 21 insertions(+), 4 deletions(-)
>>
>> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
>> index 78f5ec4e..ea9a63f 100644
>> --- a/include/linux/netdevice.h
>> +++ b/include/linux/netdevice.h
>> @@ -1498,6 +1498,7 @@ struct net_device_ops {
>>    * @IFF_FAILOVER: device is a failover master device
>>    * @IFF_FAILOVER_SLAVE: device is lower dev of a failover master device
>>    * @IFF_L3MDEV_RX_HANDLER: only invoke the rx handler of L3 master device
>> + * @IFF_LIVE_RENAME_OK: rename is allowed while device is up and running
>>    */
>>   enum netdev_priv_flags {
>>   	IFF_802_1Q_VLAN			= 1<<0,
>> @@ -1530,6 +1531,7 @@ enum netdev_priv_flags {
>>   	IFF_FAILOVER			= 1<<27,
>>   	IFF_FAILOVER_SLAVE		= 1<<28,
>>   	IFF_L3MDEV_RX_HANDLER		= 1<<29,
>> +	IFF_LIVE_RENAME_OK		= 1<<30,
>>   };
>>   
>>   #define IFF_802_1Q_VLAN			IFF_802_1Q_VLAN
>> @@ -1561,6 +1563,7 @@ enum netdev_priv_flags {
>>   #define IFF_FAILOVER			IFF_FAILOVER
>>   #define IFF_FAILOVER_SLAVE		IFF_FAILOVER_SLAVE
>>   #define IFF_L3MDEV_RX_HANDLER		IFF_L3MDEV_RX_HANDLER
>> +#define IFF_LIVE_RENAME_OK		IFF_LIVE_RENAME_OK
>>   
>>   /**
>>    *	struct net_device - The DEVICE structure.
>> diff --git a/net/core/dev.c b/net/core/dev.c
>> index 9823b77..1622d88 100644
>> --- a/net/core/dev.c
>> +++ b/net/core/dev.c
>> @@ -1185,7 +1185,21 @@ int dev_change_name(struct net_device *dev, const char *newname)
>>   	BUG_ON(!dev_net(dev));
>>   
>>   	net = dev_net(dev);
>> -	if (dev->flags & IFF_UP)
>> +
>> +	/* Some auto-enslaved devices e.g. failover slaves are
>> +	 * special, as userspace might rename the device after
>> +	 * the interface had been brought up and running since
>> +	 * the point kernel initiated auto-enslavement. Allow
>> +	 * live name change even when these slave devices are
>> +	 * up and running.
>> +	 *
>> +	 * Typically, users of these auto-enslaving devices
>> +	 * don't actually care about slave name change, as
>> +	 * they are supposed to operate on master interface
>> +	 * directly.
>> +	 */
>> +	if (dev->flags & IFF_UP &&
>> +	    likely(!(dev->priv_flags & IFF_LIVE_RENAME_OK)))
>>   		return -EBUSY;
>>   
>>   	write_seqcount_begin(&devnet_rename_seq);
>> diff --git a/net/core/failover.c b/net/core/failover.c
>> index 4a92a98..b5cd3c7 100644
>> --- a/net/core/failover.c
>> +++ b/net/core/failover.c
>> @@ -80,14 +80,14 @@ static int failover_slave_register(struct net_device *slave_dev)
>>   		goto err_upper_link;
>>   	}
>>   
>> -	slave_dev->priv_flags |= IFF_FAILOVER_SLAVE;
>> +	slave_dev->priv_flags |= (IFF_FAILOVER_SLAVE | IFF_LIVE_RENAME_OK);
>>   
>>   	if (fops && fops->slave_register &&
>>   	    !fops->slave_register(slave_dev, failover_dev))
>>   		return NOTIFY_OK;
>>   
>>   	netdev_upper_dev_unlink(slave_dev, failover_dev);
>> -	slave_dev->priv_flags &= ~IFF_FAILOVER_SLAVE;
>> +	slave_dev->priv_flags &= ~(IFF_FAILOVER_SLAVE | IFF_LIVE_RENAME_OK);
>>   err_upper_link:
>>   	netdev_rx_handler_unregister(slave_dev);
>>   done:
>> @@ -121,7 +121,7 @@ int failover_slave_unregister(struct net_device *slave_dev)
>>   
>>   	netdev_rx_handler_unregister(slave_dev);
>>   	netdev_upper_dev_unlink(slave_dev, failover_dev);
>> -	slave_dev->priv_flags &= ~IFF_FAILOVER_SLAVE;
>> +	slave_dev->priv_flags &= ~(IFF_FAILOVER_SLAVE | IFF_LIVE_RENAME_OK);
>>   
>>   	if (fops && fops->slave_unregister &&
>>   	    !fops->slave_unregister(slave_dev, failover_dev))
>> -- 
>> 1.8.3.1

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