Re: [PATCH 1/2] net: Allow to create links with given ifindex

[Pavel Emelyanov <xemul@parallels.com>]

>> First, let's imagine that on host A the loopback device got registered as
>> first device, but on host B for some reason some other device got registered
>> first. In that case after migration from A to B the lo on B will have index
>> equals 2. And there's no any strict requirement that lo's per net operations
>> are registered first. Please, correct me if I'm wrong.
> 
> Actually there is a hard requirement that the loopback device be the
> last device in a network namespace to be unregistered.  We meet that
> requirement by registering the loopback device first
> "net/core/dev.c:net_dev_init()".

Hm... Indeed, and this is good news!

>> Next. In fact, lo is not the only problem. Look at the e.g. sit versus ipgre
>> fallback devices. Both gets created on netns creation and obtain whatever
>> ifindices are generated for them. Even if we make ifidex per netns chances
>> that sit gets registered _strictly_ before ipgre equal zero, since they are
>> both modules.
> 
> True.  However those fallback devices should no longer be needed,
> and even if they are I think you can delete and recreate them.

Good idea! I will look at that direction.

> Making lo the particularly interesting case.

Yup, provided we can manually recreate those auto-created devices this solves
the issue.

>> Just an idea -- is it worth moving the possibility to have ifindidces intersect
>> under CONFIG_<SOMETHING> (EXPERT/CHECKPOINT_RESTORE) to let wider audience check
>> the code in real-life?
> 
> I think the best testing we are going to get diversity wise is to create
> a per netns counter into dev_new_index when net-next opens up.
> 
> Having an ifindex that we can only set at netdevice creation time seems
> reasonable.  

OK, thank you, Eric.

> Eric
> .
> 

Thanks,
Pavel