netdev - Re: [v2 PATCH 3/10] rhashtable: Allow hashfn to be unset

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Date:	Mon, 23 Mar 2015 09:58:42 +0000
From:	Thomas Graf <tgraf@...g.ch>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Patrick McHardy <kaber@...sh.net>,
	Josh Triplett <josh@...htriplett.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	netdev@...r.kernel.org
Subject: Re: [v2 PATCH 3/10] rhashtable: Allow hashfn to be unset

On 03/23/15 at 08:12am, Herbert Xu wrote:
> On Sun, Mar 22, 2015 at 12:32:57PM +0000, Thomas Graf wrote:
> > Sure but why not just store key_len/4 in ht->p.key_len then if you
> > opt in to jhash2() in rhashtable_init()?
> 
> Because that breaks rhashtable_compare/memcmp.

Thanks. Didn't see that.

> > > > > +	if (!__builtin_constant_p(params.key_len))
> > > > > +		hash = ht->p.hashfn(key, ht->key_len, tbl->hash_rnd);
> > > > 
> > > > I don't understand this. It looks like you only consider
> > > > params->key_len if it's constant.
> > > 
> > > If params->key_len is not constant, then params == ht->p.
> > 
> > I must be missing something obvious. Who guarantees that? I can see
> > that's true for the current callers but what prevents anybody from
> > using rhashtable_lookup_fast() with a key length not known at compile
> > time and pass it as rhashtable_params?
> 
> They shouldn't be doing that.  The whole point of this function
> is to have it inlined so all external callers of it should be
> supplying a constant parameter.  We could add a __ variant that
> is only called by rhashtable if you like so we can enforce this
> in rhashtable_lookup_fast.

If you add such constraints it must be clearly documented. There
is no way of figuring this out right now without reading the entire
rhashtable code (and talking to you).

> > I still don't get this. Why do we fall back to jhash2() if
> > params.key_len is set but not if only ht->p.key_len is set?
> 
> Because if params.key_len is not set then we have no idea whether
> we should use jhash or jhash2 because ht->p.key_len cannot be
> known at compile time.  This is only used by netfilter currently
> as it has a key-length set at run-time.

Sorry, still not getting this ;-)

nft_hash sets key_len to set->klen and passes it to rhashtable_init().
rhashtable_init() should then fall back to jhash() or jhash2() if no
hashfn is provided. Why is the logic in rht_key_hashfn() different?
Actually, in which case is ht->p.hashfn not set in rht_key_hashfn()?
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