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

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Message-ID: <20180821171555.GA16545@cmpxchg.org>
Date:   Tue, 21 Aug 2018 13:15:55 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Konstantin Khlebnikov <koct9i@...il.com>
Cc:     Matthew Wilcox <willy@...radead.org>, 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>,
        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 Tue, Aug 21, 2018 at 08:11:44AM +0300, Konstantin Khlebnikov wrote:
> 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.

It is nr_pages * 200 * recent_scanned, where recent_scanned can be up
to four times of what's on the LRUs. That can overflow a u32 with even
small amounts of memory.