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Date: Thu, 14 Feb 2008 20:18:02 +0200
From: "Pekka Enberg" <penberg@cs.helsinki.fi>
To: "Nishanth Aravamudan" <nacc@us.ibm.com>
Subject: Re: [PATCH 2/6] fs/direct-io.c: Use DIV_ROUND_UP
Cc: "Julia Lawall" <julia@diku.dk>, linux-kernel@vger.kernel.org,
       kernel-janitors@vger.kernel.org
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Hi Nishanth,

On Thu, Feb 14, 2008 at 7:38 PM, Nishanth Aravamudan <nacc@us.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.

                                      Pekka