netdev - Re: [RFC PATCH net-next] failover: allow name change on IFF

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Date:   Wed, 6 Mar 2019 02:23:31 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     si-wei liu <si-wei.liu@...cle.com>
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        Sridhar Samudrala <sridhar.samudrala@...el.com>,
        Jakub Kicinski <kubakici@...pl>, Jiri Pirko <jiri@...nulli.us>,
        David Miller <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org, liran.alon@...cle.com,
        boris.ostrovsky@...cle.com, vijay.balakrishna@...cle.com
Subject: Re: [RFC PATCH net-next] failover: allow name change on IFF_UP slave
 interfaces

On Tue, Mar 05, 2019 at 11:15:06PM -0800, si-wei liu wrote:
> 
> 
> On 3/5/2019 10:43 PM, Michael S. Tsirkin wrote:
> > On Tue, Mar 05, 2019 at 04:51:00PM -0800, si-wei liu wrote:
> > > 
> > > On 3/5/2019 4:36 PM, Michael S. Tsirkin wrote:
> > > > On Tue, Mar 05, 2019 at 04:20:50PM -0800, si-wei liu wrote:
> > > > > On 3/5/2019 4:06 PM, Michael S. Tsirkin wrote:
> > > > > > On Tue, Mar 05, 2019 at 11:35:50AM -0800, si-wei liu wrote:
> > > > > > > On 3/5/2019 11:24 AM, Stephen Hemminger wrote:
> > > > > > > > On Tue, 5 Mar 2019 11:19:32 -0800
> > > > > > > > si-wei liu <si-wei.liu@...cle.com> wrote:
> > > > > > > > 
> > > > > > > > > > I have a vague idea: would it work to *not* set
> > > > > > > > > > IFF_UP on slave devices at all?
> > > > > > > > > Hmm, I ever thought about this option, and it appears this solution is
> > > > > > > > > more invasive than required to convert existing scripts, despite the
> > > > > > > > > controversy of introducing internal netdev state to differentiate user
> > > > > > > > > visible state. Either we disallow slave to be brought up by user, or to
> > > > > > > > > not set IFF_UP flag but instead use the internal one, could end up with
> > > > > > > > > substantial behavioral change that breaks scripts. Consider any admin
> > > > > > > > > script that does `ip link set dev ... up' successfully just assumes the
> > > > > > > > > link is up and subsequent operation can be done as usual.
> > > > > > How would it work when carrier is off?
> > > > > > 
> > > > > > > While it *may*
> > > > > > > > > work for dracut (yet to be verified), I'm a bit concerned that there are
> > > > > > > > > more scripts to be converted than those that don't follow volatile
> > > > > > > > > failover slave names. It's technically doable, but may not worth the
> > > > > > > > > effort (in terms of porting existing scripts/apps).
> > > > > > > > > 
> > > > > > > > > Thanks
> > > > > > > > > -Siwei
> > > > > > > > Won't work for most devices.  Many devices turn off PHY and link layer
> > > > > > > > if not IFF_UP
> > > > > > > True, that's what I said about introducing internal state for those driver
> > > > > > > and other kernel component. Very invasive change indeed.
> > > > > > > 
> > > > > > > -Siwei
> > > > > > Well I did say it's vague.
> > > > > > How about hiding IFF_UP from dev_get_flags (and probably
> > > > > > __dev_change_flags)?
> > > > > > 
> > > > > Any different? This has small footprint for the kernel change for sure,
> > > > > while the discrepancy is still there. Anyone who writes code for IFF_UP will
> > > > > not notice IFF_FAILOVER_SLAVE.
> > > > > 
> > > > > Not to mention more userspace "fixup" work has to be done due to this
> > > > > change.
> > > > > 
> > > > > -Siwei
> > > > > 
> > > > > 
> > > > Point is it's ok since most userspace should just ignore slaves
> > > > - hopefully it will just ignore it since it already
> > > > ignores interfaces that are down.
> > > Admin script thought the interface could be bright up and do further
> > > operations without checking the UP flag.
> > These scripts then would be broken  on any box with multiple interfaces
> > since not all of these would have carrier.
> Consider a script executing `ifconfig ... up' and once succeeds runs tcpdump
> or some other command relying on UP interface. It's quite common that those
> scripts don't check the UP flag but instead just rely on the well-known fact
> that the command exits with 0 meaning the interface should be UP. This
> change might well break scripts of that kind.

I am sorry I don't get it. Could you give an example
of a script that works now but would be broken?


> > 
> > 
> > > It doesn't look to be a reliable
> > > way of prohibit userspace from operating against slaves.
> > > 
> > > -Siwei
> > > 
> > > 
> > This does not mean we shouldn't make an effort to disable broken
> > configurations.
> > 
> > I am not arguing against your patch. Not at all. I see better
> > hiding of slaves as a separate enhancement.
> I understand, but my point is we should try to minimize unnecessary side
> impact to the current usage for whatever "hiding" effort we can make. It's
> hard to find a tradeoff sometimes.

Yes if some userspace made an assumption and it worked, we should keep
it working I think. I don't necessarily agree we should worry too much
about theoretical issues. In half a year since the feature got merged
it's unlikely there are millions of slightly different scripts using it.

> > 
> > 
> > Acked-by: Michael S. Tsirkin <mst@...hat.com>
> > 
> > 
> Thank you.
> 
> -Siwei