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

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

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.

                                      Pekka
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