netdev - Re: [PATCHv4 net-next-2.6 1/5] XFRM,IPv6: Remove xfrm_spi

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Date:	Tue, 5 Oct 2010 10:11:14 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Arnaud Ebalard <arno@...isbad.org>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	netdev@...r.kernel.org
Subject: Re: [PATCHv4 net-next-2.6 1/5] XFRM,IPv6: Remove xfrm_spi_hash()
	dependency on destination address

On Mon, Oct 04, 2010 at 10:51:38PM +0200, Arnaud Ebalard wrote:
> 
> Herbert Xu <herbert@...dor.apana.org.au> writes:
>
> > I'm fine with doing this for inbound SAs.  However, I can't see
> > how we can do this for outbound SAs where the SPI is chosen by
> > the remote end.
> 
> The change *does not* make the lookup in the hash table rely only on the
> spi, i.e. __xfrm_state_lookup() is still passed the address. It only
> removes the destination address from the computation of the hash. This
> allows passing NULL to __xfrm_state_lookup() specifically for input path
> and make the lookup only based on the SPI. The destination address check
> is done later (possibly after IRO remapping).
> 
> Except if I really missed something, this has no impact on outbound SA
> (other hashtables are used in that case). 

I'm thinking about the case where each remote end (or one remote
end with many IP addresses) chooses to use a single SPI which then
all gets hashed to the same bucket.

Outbound SAs are hashed into the same SPI hash table as inbound SAs.

> > Incidentally, it appears that our hash could do with some
> > strengthening.
> 
> After the change, xfrm_spi_hash() would contain:
> 
>  	unsigned int h = (__force u32)spi ^ proto;
>         return  ((h ^ (h >> 10) ^ (h >> 20)) & hmask)
> 
> which seems to spread the bits h correctly into hmask bits (I mean for
> the effort ;-) ). Are you thinking about something like changing the
> shifts based on the length of the mask?

What I meant here is that even without your change, it is relatively
easy to cause many SAs to be hashed to the same bucket.

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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