netdev - Re: [PATCH] ipv4: Remove output route check in ipv4

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Message-ID: <alpine.LFD.2.00.1301162236320.1760@ja.ssi.bg>
Date:	Wed, 16 Jan 2013 23:01:39 +0200 (EET)
From:	Julian Anastasov <ja@....bg>
To:	Steffen Klassert <steffen.klassert@...unet.com>
cc:	David Miller <davem@...emloft.net>, timo.teras@....fi,
	luky-37@...mail.com, pupilla@...ero.it, netdev@...r.kernel.org
Subject: Re: [PATCH] ipv4: Remove output route check in ipv4_mtu


	Hello,

On Wed, 16 Jan 2013, Steffen Klassert wrote:

> On Wed, Jan 16, 2013 at 10:58:13AM +0200, Julian Anastasov wrote:
> > 
> > 	This fix looks good to me. But I see that we
> > have another problem here. doc/ip-cref.tex in iproute2
> > claims that a locked MTU value has priority and
> > PMTU should not be considered.
> > 
> > 	IIRC, rt_pmtu is valid only for output routes but
> > we do not check the lock flag here. What about such
> > variant:
> > 
> > static unsigned int ipv4_mtu(const struct dst_entry *dst)
> > {
> > 	const struct rtable *rt = (const struct rtable *) dst;
> > 	unsigned int mtu = rt->rt_pmtu;
> > 
> > 	if (unlikely(dst_metric_locked(dst, RTAX_MTU))) {
> > 		mtu = dst_metric_raw(dst, RTAX_MTU);
> > 		if (!mtu) {
> > 			mtu = dst->dev->mtu;
> > 			if (rt->rt_uses_gateway && mtu > 576)
> > 				mtu = 576;
> > 		}
> > 	} else if (!mtu || time_after_eq(jiffies, rt->dst.expires)) {
> > 		mtu = dst_metric_raw(dst, RTAX_MTU);
> > 		if (!mtu)
> > 			mtu = dst->dev->mtu;
> > 	}
> > 	if (mtu > IP_MAX_MTU)
> > 		mtu = IP_MAX_MTU;
> > 
> > 	return mtu;
> > }
> > 
> > 	I.e. order becomes:
> > 
> > mtu lock non-zero => fixed fib_mtu
> > mtu lock 0 => device MTU, up to 576 if via GW, ignore PMTU
> > PMTU (output routes)
> > mtu non-zero => fib_mtu
> > device MTU
> 
> If this is the desired order, I'm fine with the above variant.
> 
> > 
> > 	Also, it seems __ip_rt_update_pmtu should start
> > with such dst_metric_locked(dst, RTAX_MTU) check?
> > 
> 
> Not absolutely sure what you want to do if the mtu is locked.
> But if you don't want to genetate a nh exception and don't update
> rt->rt_pmtu in this case, rt->rt_pmtu is never set on routes with
> locked mtu. We probably would not need to change ipv4_mtu() to the
> above variant.

	Yes, we have to exit __ip_rt_update_pmtu if
fib_mtu is locked. Then rt_pmtu should stay 0 and
your variant of ipv4_mtu should be ok. I hope you
can simply extend your patch with such additional
check in __ip_rt_update_pmtu:

	if (dst_metric_locked(dst, RTAX_MTU))
		return;

Regards

--
Julian Anastasov <ja@....bg>
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