Re: [virtio-dev] Re: net_failover slave udev renaming (was Re: [RFC PATCH net-next v6 4/4] netvsc: refactor notifier/event handling code to use the bypass framework)

[si-wei liu <si-wei.liu@oracle.com>]

On 2/28/2019 6:26 AM, Michael S. Tsirkin wrote:
> On Thu, Feb 28, 2019 at 01:32:12AM -0800, si-wei liu wrote:
> > > > Will the
> > > > change break userspace further?
> > > >
> > > > -Siwei
> > > Didn't you show userspace is already broken. You can't "further
> > > break it", rename already fails.
> > It's a race, userspace tends to give slave a user(space) desired name but
> > sometimes may fail due to this race. Today if failover master is not up,
> > rename would succeed anyway. While what you proposed prohibits user from
> > providing a name in all circumstances if I understand you correctly. That's
> > what I meant of breaking userspace further. On the other hand, you seem to
> > tighten the kernel default naming to udev predictable names, which is
> > derived from only recent systemd-udevd, while there exists many possible
> > userspace naming schemes out of that. Users today who deliberately chooses
> > to disable predictable naming (net.ifnames=0 biosdevname=0) and fall back to
> > kernel provided names would expect the ethX pattern, with this change
> > admin/user scripts which matches the ethX pattern could potentially break.
> Whatever crashes with a name not matching ethX will crash on the
> standby interface *anyway*.
With udev predictable naming disabled they should not. It's not hard for 
user to look for device attribute to persistent the name well, in a 
consistent and reliable way.

> So I think what you are saying is that someone might have already
> written scripts and gotten them to work on v4.17 when STANDBY was
> included and these scripts rely on ethX. Now these scripts
> will break.
The controversial part is the new kernel naming pattern. Initially I 
thought there shouldn't be such crazy scripts relying on the pattern, 
but when I worked on cloud-init it I realized that there's already a lot 
of software taking assumption around the 'eth0' name. In the past I've 
seen random scripts that parses the ethX name assumes (incorrectly) the 
name ends up with digits, or even the digits and name are 1:1 mapped. Of 
course, you can say these are bugs in scripts themselves.

Anyway, I'll let others in the netdev to comment on this new scheme, 
maybe that's the concern of merely myself. The good part of your 
proposal is that we can get consistent slave name, which still plays its 
role until we move towards making slave names less relevant, i.e. 
ideally a 1-netdev model. I think we both agree that the master matters 
more than the slave names.

> Maybe it is still early enough (just half a year passed) that the
> number of these users would be small.  So how about a kernel config
> option and maybe a module parameter to rename the primary?  People can
> then opt in to the old broken behaviour.
Were I could I would ask  why a similar opt-in (kernel config or module 
parameter) couldn't be implemented to open up the rename restriction on 
slave, net_failover in particular. What I felt about this rename 
restriction was more because of historical reason than anything else, 
while net_failover is comparatively a new type of link that we are now 
designing proper use case it should support, and can get it shaped to 
whatever it fits. My personal view is that the slave can't be renamed 
when master is running is just implementation details that got 
incorrectly exposed to userspace apps for many years. It's old behavior 
with historical reason for sure, but I don't think this applies to 
net_failover.

(FWIW as one previous bond maintainer for another OS, we relieved the 
rename restriction slaves 13 year ago, while no single complaint or 
issue was ever raised because of this change over the years, neither 
from the customers of tens of millions of installation base, nor the 
FOSS software running atop. Of course, Linux is different so that 
experience doesn't count.)

Thanks,
-Siwei