netdev - Re: [PATCH 5/5] tun: vringfd xmit support.

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Date:	Fri, 18 Apr 2008 07:25:27 -0700
From:	"Ray Lee" <ray-lk@...rabbit.org>
To:	"pradeep singh rautela" <rautelap@...il.com>
Cc:	"Rusty Russell" <rusty@...tcorp.com.au>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"Max Krasnyansky" <maxk@...lcomm.com>,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 5/5] tun: vringfd xmit support.

On Fri, Apr 18, 2008 at 4:46 AM, pradeep singh rautela
<rautelap@...il.com> wrote:
>
> On Fri, Apr 18, 2008 at 10:13 AM, Rusty Russell <rusty@...tcorp.com.au> wrote:
>  >  +               /* How many pages will this take? */
>  >  +               npages = 1 + (base + len - 1)/PAGE_SIZE - base/PAGE_SIZE;
>
>  Hi Rusty,
>  A trivial suggestion, how about
>           npages = 1+(len -1)/PAGE_SIZE ?

That's not the same. In particular, his version accounts for the
fractional page at the beginning, while yours doesn't.

While it's tempting to use algebra to simplify things, it's not safe
to do so when the expression involves division over the integers. The
only care-free integer math in a computer is subtraction and ++.
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