From mboxrd@z Thu Jan  1 00:00:00 1970
From: Thomas Graf <tgraf@suug.ch>
Subject: Re: [PATCH net-next v2] rhashtable: Warn if min_size or max_size are
 not a power of two
Date: Thu, 19 Mar 2015 21:15:46 +0000
Message-ID: <20150319211546.GA4190@casper.infradead.org>
References: <29119e13cd16d4a98b4ed6806bec2683943bcd5f.1426782799.git.tgraf@suug.ch>
 <20150319.152450.1573636653085091157.davem@davemloft.net>
 <20150319194608.GA27962@casper.infradead.org>
 <20150319210239.GB9601@gondor.apana.org.au>
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Cc: David Miller <davem@davemloft.net>, netdev@vger.kernel.org
To: Herbert Xu <herbert@gondor.apana.org.au>
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On 03/20/15 at 08:02am, Herbert Xu wrote:
> On Thu, Mar 19, 2015 at 07:46:08PM +0000, Thomas Graf wrote:
> > The current code correctly limits table size to the next power of two.
> > This check is solely to catch programming errors.
> > 
> > Signed-off-by: Thomas Graf <tgraf@suug.ch>
> 
> I don't see the point of this.  A maximum size of 3 says that
> the table size should never exceed 3 which makes perfect sense.
> And our current code will respect that.
> 
> So why force it to be a power of 2 just because our table sizes
> happen to be powers of 2?

rht_grow_above_75() checks the old table size:

(!ht->p.max_size || tbl->size < ht->p.max_size);

If you specify max_size = 3, it grows the table to tbl->size = 4.
This can be avoided if max_size is a power of two.