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

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Message-ID: <20201116132051.4eef4247@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Mon, 16 Nov 2020 13:20:51 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     "Mahesh Bandewar (महेश बंडेवार)" <maheshb@...gle.com>
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 Mon, 16 Nov 2020 12:50:22 -0800 Mahesh Bandewar (महेश बंडेवार) wrote:
> On Mon, Nov 16, 2020 at 12:17 PM Jakub Kicinski <kuba@...nel.org> wrote:
> > On Mon, 16 Nov 2020 12:02:48 -0800 Mahesh Bandewar (महेश बंडेवार) wrote:  
> > > 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.  
> >
> > I must be confused. Are all namespaces spawned off init_net, not the
> > current netns the process is in?  
> The namespace hierarchy is maintained in user-ns while we have per-ns
> sysctls hanging off of a netns object and we don't have parent (netns)
> reference when initializing newly created netns values. One can copy
> the current value of the settings from root-ns but I don't think
> that's a good practice since there is no clear way to affect those
> values when the root-ns changes them. Also from the isolation
> perspective (I think) it's better to have this behavior (sysctl
> values) stand on it's own i.e. have default values and then alter
> values on it's own without linking to any other netns values.

To be clear, what I meant was just to make the sysctl per namespace.
That way you can deploy a workload with this sysctl set appropriately
without changing the system-global setting.

Is your expectation that particular application stacks would take
advantage of this, or that people would set this to 1 across the
fleet?