netdev - Re: [PATCH net-next 2/2] reciprocal_divide: correction/update of the algorithm

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Date:	Thu, 16 Jan 2014 11:26:18 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Daniel Borkmann <dborkman@...hat.com>, davem@...emloft.net,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Austin S Hemmelgarn <ahferroin7@...il.com>,
	Jesse Gross <jesse@...ira.com>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Matt Mackall <mpm@...enic.com>,
	Pekka Enberg <penberg@...nel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Andy Gospodarek <andy@...yhouse.net>,
	Veaceslav Falico <vfalico@...hat.com>,
	Jay Vosburgh <fubar@...ibm.com>,
	Jakub Zawadzki <darkjames-ws@...kjames.pl>
Subject: Re: [PATCH net-next 2/2] reciprocal_divide: correction/update of the algorithm

Hi Eric!

On Wed, Jan 15, 2014 at 07:07:26PM -0800, Eric Dumazet wrote:
> On Thu, 2014-01-16 at 00:23 +0100, Daniel Borkmann wrote:
> 
> > Also, reciprocal_value() and reciprocal_divide() always return 0
> > for divisions by 1. This is a bit worrisome as those functions
> > also get used in mm/slab.c and lib/flex_array.c, apparently for
> > index calculation to access array slots. 
> 
> Hi Daniel
> 
> This off-by-one limitation is a known one,
> and mm/slab.c does not have an issue with it because :
> 
> - Minimal object size is not 1 byte, but 8 (or maybe 4)
> - We always divide a multiple of the divisor,
>   so there is no off-by-one effect.
> 
> Little attached prog does a brute force check if needed.
> 
> So far, the only relevant issue was about BPF, and a better
> documentation of reciprocal_divide() use cases.
> 
> (I let Jesse comment on the flex_array case)
> 
> I am unsure we want to 'fix' things, we tried hard in the past to avoid
> divides, so the ones we use are usually because the divisor is not
> constant, so the reciprocal doesn't help.
> 
> (BPF is fixed in David tree)
> 
> Thanks !

You are right, we rewrite that part. The text is still from the first commit
message where we did no full impact analysis. ;)

Thanks,

  Hannes

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