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Message-ID: <Pine.LNX.4.64.0802142049510.19977@ask.diku.dk>
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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