netdev - Re: Question on rhashtable in worst-case scenario.

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Date:	Wed, 30 Mar 2016 21:55:21 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Johannes Berg <johannes@...solutions.net>
Cc:	Ben Greear <greearb@...delatech.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
	netdev <netdev@...r.kernel.org>, Thomas Graf <tgraf@...g.ch>
Subject: Re: Question on rhashtable in worst-case scenario.

On Wed, Mar 30, 2016 at 11:14:12AM +0200, Johannes Berg wrote:
> On Tue, 2016-03-29 at 09:16 -0700, Ben Greear wrote:
> > Looks like rhashtable has too much policy in it to properly deal with
> > cases where there are too many hash collisions, so I am going to work
> > on reverting it's use in mac80211.
> 
> I'm not really all that happy with that approach - can't we fix the
> rhashtable? It's a pretty rare corner case that many keys really are
> identical and no kind of hash algorithm, but it seems much better to
> still deal with it than to remove the rhashtable usage and go back to
> hand-rolling something.

Well to start with you should assess whether you really want to
hash multiple objects with the same key.  In particular, can an
adversary generate a large number of such objects?

If your conclusion is that yes you really want to do this, then
we have the parameter insecure_elasticity that you can use to
disable the rehashing based on chain length.

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