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

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Message-ID: <20180818012213.GA14115@bombadil.infradead.org>
Date:   Fri, 17 Aug 2018 18:22:13 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Roman Gushchin <guro@...com>
Cc:     linux-mm@...ck.org, 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>,
        Konstantin Khlebnikov <koct9i@...il.com>
Subject: Re: [PATCH RFC] mm: don't miss the last page because of round-off
 error

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.