netdev - Re: [v2 PATCH 7/10] rhashtable: Disable automatic shrinking

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Message-ID: <20150323093952.GA12635@gondor.apana.org.au>
Date:	Mon, 23 Mar 2015 20:39:52 +1100
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Thomas Graf <tgraf@...g.ch>
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 7/10] rhashtable: Disable automatic shrinking

On Mon, Mar 23, 2015 at 09:36:32AM +0000, Thomas Graf wrote:
>
> > You never want automatic shrinking unless all your users are
> > trusted.  I doubt there would be many rhashtable users where
> > this would apply.  Even nft_hash is quite tenuous.
> 
> Why?

Because with multiple rehashing it's quite easy to convert your
hash table into a linked list by repeatedly growing and shrinking.

Multiple rehashing simply cannot work unless you get rid of automatic
shrinking for the untrusted case.

> Same can be said for growing. Why do we differ between the two?
> Would you expect users requiring shrinking() to call
> rhashtable_shrink() after every remove? Should they encode their
> own logic based on rhashtable internals?

No we must be able to grow or insertions will fail.  The same cannot
be said for shrinking.

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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