netdev - Re: [PATCH net] ipv4: lock mtu in fnhe when received PMTU < net.ipv4.route.min

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Message-ID: <20180312160528.GA30326@bistromath.localdomain>
Date:   Mon, 12 Mar 2018 17:05:28 +0100
From:   Sabrina Dubroca <sd@...asysnail.net>
To:     David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, sbrivio@...hat.com
Subject: Re: [PATCH net] ipv4: lock mtu in fnhe when received PMTU <
 net.ipv4.route.min_pmtu

2018-03-09, 16:06:19 -0500, David Miller wrote:
> From: Sabrina Dubroca <sd@...asysnail.net>
> Date: Fri,  9 Mar 2018 17:43:21 +0100
> 
> > diff --git a/include/net/ip_fib.h b/include/net/ip_fib.h
> > index f80524396c06..77d0a78cf7d2 100644
> > --- a/include/net/ip_fib.h
> > +++ b/include/net/ip_fib.h
> > @@ -59,6 +59,7 @@ struct fib_nh_exception {
> >  	int				fnhe_genid;
> >  	__be32				fnhe_daddr;
> >  	u32				fnhe_pmtu;
> > +	bool				fnhe_mtu_locked;
> >  	__be32				fnhe_gw;
> >  	unsigned long			fnhe_expires;
> >  	struct rtable __rcu		*fnhe_rth_input;
> > diff --git a/include/net/route.h b/include/net/route.h
> > index 1eb9ce470e25..729bb5e61e9a 100644
> > --- a/include/net/route.h
> > +++ b/include/net/route.h
> > @@ -64,6 +64,7 @@ struct rtable {
> >  
> >  	/* Miscellaneous cached information */
> >  	u32			rt_pmtu;
> > +	bool			rt_mtu_locked;
> >  
> >  	u32			rt_table_id;
> >  
> 
> Please use a flag bit for this, we've worked hard to shrink these
> datastructures as much as possible.

Oops, sorry.

> I think if you just choose an unused RTCF_* bit (f.e. 0x02000000) for
> the state, you can use that because values propagate into the
> rtable->rt_flags, and do not propagate out.  So you should be able to
> use it in this way privately inside the kernel.

What about a bitfield?

-	u32			rt_pmtu;
+	u32			rt_mtu_locked:1,
+				rt_pmtu:31;

Since it's going to be private to the kernel, I'd rather not use a
value that's in uapi, especially considering that they're almost all
used (unless we start recycling).

-- 
Sabrina