Re: [PATCH net-next v4 5/6] net: bridge: Add a configurable default FDB learning limit

[Johannes Nixdorf]

On Wed, Sep 20, 2023 at 02:00:27PM +0300, Nikolay Aleksandrov wrote:
> On 9/19/23 11:12, Johannes Nixdorf wrote:
> > Add a Kconfig option to configure a default FDB learning limit system
> > wide, so a distributor building a special purpose kernel can limit all
> > created bridges by default.
> > 
> > The limit is only a soft default setting and overrideable on a per bridge
> > basis using netlink.
> > 
> > Signed-off-by: Johannes Nixdorf <jnixdorf-oss@xxxxxx>
> > ---
> >   net/bridge/Kconfig     | 13 +++++++++++++
> >   net/bridge/br_device.c |  2 ++
> >   2 files changed, 15 insertions(+)
> > 
> > diff --git a/net/bridge/Kconfig b/net/bridge/Kconfig
> > index 3c8ded7d3e84..c0d9c08088c4 100644
> > --- a/net/bridge/Kconfig
> > +++ b/net/bridge/Kconfig
> > @@ -84,3 +84,16 @@ config BRIDGE_CFM
> >   	  Say N to exclude this support and reduce the binary size.
> >   	  If unsure, say N.
> > +
> > +config BRIDGE_DEFAULT_FDB_MAX_LEARNED
> > +	int "Default FDB learning limit"
> > +	default 0
> > +	depends on BRIDGE
> > +	help
> > +	  Sets a default limit on the number of learned FDB entries on
> > +	  new bridges. This limit can be overwritten via netlink on a
> > +	  per bridge basis.
> > +
> > +	  The default of 0 disables the limit.
> > +
> > +	  If unsure, say 0.
> > diff --git a/net/bridge/br_device.c b/net/bridge/br_device.c
> > index 9a5ea06236bd..3214391c15a0 100644
> > --- a/net/bridge/br_device.c
> > +++ b/net/bridge/br_device.c
> > @@ -531,6 +531,8 @@ void br_dev_setup(struct net_device *dev)
> >   	br->bridge_ageing_time = br->ageing_time = BR_DEFAULT_AGEING_TIME;
> >   	dev->max_mtu = ETH_MAX_MTU;
> > +	br->fdb_max_learned = CONFIG_BRIDGE_DEFAULT_FDB_MAX_LEARNED;
> > +
> >   	br_netfilter_rtable_init(br);
> >   	br_stp_timer_init(br);
> >   	br_multicast_init(br);
> > 
> 
> This one I'm not sure about at all. Distributions can just create the bridge
> with a predefined limit. This is not flexible and just adds
> one more kconfig option that is rather unnecessary. Why having a kconfig
> knob is better than bridge creation time limit setting? You still have
> to create the bridge, so why not set the limit then?

The problem I'm trying to solve here are unaware applications. Assuming
this change lands in the next Linux release there will still be quite
some time until the major applications that create bridges (distribution
specific or common network management tools, the container solution of
they day, for embedded some random vendor tools, etc.) will pick it
up. In this series I chose a default of 0 to not break existing setups
that rely on some arbitrary amount of FDB entries, so those unaware
applications will create bridges without limits. I added the Kconfig
setting so someone who knows their use cases can still set a more fitting
default limit.

More specifically to our use case as an embedded vendor that builds their
own kernels and knows they have no use case that requires huge FDB tables,
the kernel config allows us to set a safe default limit before starting
to teach all our applications and our upstream vendors' code about the
new netlink attribute. As this patch is relatively simple, we can also
keep it downstream if there is opposition to it here though.