linux-kernel - Re: [PATCH 2/6] fs/direct-io.c: Use DIV_ROUND

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Date:	Thu, 14 Feb 2008 20:50:39 +0100 (CET)
From:	Julia Lawall <julia@...u.dk>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Nishanth Aravamudan <nacc@...ibm.com>,
	linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH 2/6] fs/direct-io.c: Use DIV_ROUND_UP

On Thu, 14 Feb 2008, Pekka Enberg wrote:

> Hi Nishanth,
> 
> On Thu, Feb 14, 2008 at 7:38 PM, Nishanth Aravamudan <nacc@...ibm.com> wrote:
> >  Is it just me, or does
> >
> >         ((user_addr + iov[seg].iov_len + PAGE_SIZE - 1)/PAGE_SIZE - user_addr/PAGE_SIZE)
> >
> >  not simplify to
> >
> >         = ((iov[seg].iov_len + PAGE_SIZE - 1)/PAGE_SIZE + user_addr/PAGE_SIZE - user_addr/PAGE_SIZE)
> >
> >         = ((iov[seg].iov_len + PAGE_SIZE - 1)/PAGE_SIZE)
> >
> >         = DIV_ROUND_UP(iov[seg].iov_len, PAGE_SIZE)
> >
> >  CMIIW.
> 
> I double-checked this and I believe you're correct. It's simpler to
> see when you do:
> 
>   x = user_addr
>   y = iov[seg].iov_len
>   z = PAGE_SIZE
> 
> So
> 
>   (x + y + z - 1)/z - x/z
> 
>   = [x + (y + z - 1)]/z - x/z
> 
>   = [xz + z(y + z - 1)]/z^2 - x/z
> 
>   = x/z + (y + z - 1)/z - x/z
> 
> And the rest follows from your simplifications.

It doesn't work:

 ((3+4+5-1)/5) - (3/5) = 2
 ((4+5-1)/5) = 1

julia

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