netdev - Re: [PATCH net-next] net-loopback: allow lo dev initial state to be controlled

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Date:   Mon, 16 Nov 2020 12:02:48 -0800
From:   Mahesh Bandewar (महेश बंडेवार) 
        <maheshb@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Jian Yang <jianyang.kernel@...il.com>,
        David Miller <davem@...emloft.net>,
        linux-netdev <netdev@...r.kernel.org>,
        Jian Yang <jianyang@...gle.com>
Subject: Re: [PATCH net-next] net-loopback: allow lo dev initial state to be controlled

On Sat, Nov 14, 2020 at 10:17 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Wed, 11 Nov 2020 12:43:08 -0800 Jian Yang wrote:
> > From: Mahesh Bandewar <maheshb@...gle.com>
> >
> > Traditionally loopback devices comes up with initial state as DOWN for
> > any new network-namespace. This would mean that anyone needing this
> > device (which is mostly true except sandboxes where networking in not
> > needed at all), would have to bring this UP by issuing something like
> > 'ip link set lo up' which can be avoided if the initial state can be set
> > as UP. Also ICMP error propagation needs loopback to be UP.
> >
> > The default value for this sysctl is set to ZERO which will preserve the
> > backward compatible behavior for the root-netns while changing the
> > sysctl will only alter the behavior of the newer network namespaces.
>
> Any reason why the new sysctl itself is not per netns?
>
Making it per netns would not be very useful since its effect is only
during netns creation.

> > +netdev_loopback_state
> > +---------------------
>
> loopback_init_state ?
>
That's fine, thanks for the suggestion.

> > +Controls the loopback device initial state for any new network namespaces. By
> > +default, we keep the initial state as DOWN.
> > +
> > +If set to 1, the loopback device will be brought UP during namespace creation.
> > +This will only apply to all new network namespaces.
> > +
> > +Default : 0  (for compatibility reasons)
> > +
> >  netdev_max_backlog
> >  ------------------
> >
> > diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
> > index a1c77cc00416..76dc92ac65a2 100644
> > --- a/drivers/net/loopback.c
> > +++ b/drivers/net/loopback.c
> > @@ -219,6 +219,13 @@ static __net_init int loopback_net_init(struct net *net)
> >
> >       BUG_ON(dev->ifindex != LOOPBACK_IFINDEX);
> >       net->loopback_dev = dev;
> > +
> > +     if (sysctl_netdev_loopback_state) {
> > +             /* Bring loopback device UP */
> > +             rtnl_lock();
> > +             dev_open(dev, NULL);
> > +             rtnl_unlock();
> > +     }
>
> The only concern I have here is that it breaks notification ordering.
> Is there precedent for NETDEV_UP to be generated before all pernet ops
> ->init was called?
I'm not sure if any and didn't see any issues in our usage / tests.
I'm not even sure anyone is watching/monitoring for lo status as such.