netdev - Re: [PATCH net 1/2] tcp: Limit number of segments generated by GSO per skb

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Date:	Mon, 30 Jul 2012 20:41:10 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Ben Greear <greearb@...delatech.com>
CC:	David Miller <davem@...emloft.net>, <netdev@...r.kernel.org>,
	<linux-net-drivers@...arflare.com>
Subject: Re: [PATCH net 1/2] tcp: Limit number of segments generated by GSO
 per skb

On Mon, 2012-07-30 at 10:23 -0700, Ben Greear wrote:
> On 07/30/2012 10:16 AM, Ben Hutchings wrote:
> > A peer (or local user) may cause TCP to use a nominal MSS of as little
> > as 88 (actual MSS of 76 with timestamps).  Given that we have a
> > sufficiently prodigious local sender and the peer ACKs quickly enough,
> > it is nevertheless possible to grow the window for such a connection
> > to the point that we will try to send just under 64K at once.  This
> > results in a single skb that expands to 861 segments.
> >
> > In some drivers with TSO support, such an skb will require hundreds of
> > DMA descriptors; a substantial fraction of a TX ring or even more than
> > a full ring.  The TX queue selected for the skb may stall and trigger
> > the TX watchdog repeatedly (since the problem skb will be retried
> > after the TX reset).  This particularly affects sfc, for which the
> > issue is designated as CVE-2012-3412.  However it may be that some
> > hardware or firmware also fails to handle such an extreme TSO request
> > correctly.
> >
> > Therefore, limit the number of segments per skb to 100.  This should
> > make no difference to behaviour unless the actual MSS is less than
> > about 700.
> 
> Please do not do this...or at least allow over-rides.  We love
> the trick of seting very small MSS and making the NICs generate
> huge numbers of small TCP frames with efficient user-space
> logic.   We use this for stateful TCP load testing when high
> numbers of tcp packets-per-second is desired.

Please test whether this actually makes a difference - my suspicion is
that 100 segments per skb is easily enough to prevent the host being a
bottleneck.

> Intel NICs, including 10G, work just fine with minimal MSS
> in this scenario.

I'll leave this to the Intel maintainers to answer.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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