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

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Date:   Wed, 22 Aug 2018 10:50:12 -0700
From:   Roman Gushchin <guro@...com>
To:     Konstantin Khlebnikov <koct9i@...il.com>
CC:     Johannes Weiner <hannes@...xchg.org>,
        Matthew Wilcox <willy@...radead.org>, <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 Wed, Aug 22, 2018 at 09:01:19AM +0300, Konstantin Khlebnikov wrote:
> On Tue, Aug 21, 2018 at 8:15 PM, Johannes Weiner <hannes@...xchg.org> wrote:
> > 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.
> 
> Ah, this thing is inverted because it aims to proportional reactivation rate
> rather than the proportional pressure to reclaimable pages.
> That's not obvious. I suppose this should be in comment above it.
> 
> Well, at least denominator should fit into unsigned long. So full
> 64/64 division is redundant.

In any case it's not related to the original issue and should be
treated separately. I'd like to keep the patch simple to make
backporting to stable easy.

All refactorings can be done separately, if necessarily.

Thanks!