linux-kernel - Re: [PATCH RFC] mm: don't miss the last page because of round-off error

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Date:   Tue, 21 Aug 2018 08:11:44 +0300
From:   Konstantin Khlebnikov <koct9i@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Roman Gushchin <guro@...com>, linux-mm@...ck.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kernel-team@...com, Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, Tejun Heo <tj@...nel.org>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH RFC] mm: don't miss the last page because of round-off error

On Sat, Aug 18, 2018 at 4:22 AM, Matthew Wilcox <willy@...radead.org> wrote:
> On Fri, Aug 17, 2018 at 04:18:34PM -0700, Roman Gushchin wrote:
>> -                     scan = div64_u64(scan * fraction[file],
>> -                                      denominator);
>> +                     if (scan > 1)
>> +                             scan = div64_u64(scan * fraction[file],
>> +                                              denominator);
>
> Wouldn't we be better off doing a div_round_up?  ie:
>
>         scan = div64_u64(scan * fraction[file] + denominator - 1, denominator);
>
> although i'd rather hide that in a new macro in math64.h than opencode it
> here.

All numbers here should be up to nr_pages * 200 and fit into unsigned long.
I see no reason for u64. If they overflow then u64 wouldn't help either.

There is macro DIV_ROUND_UP in kernel.h