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

Matthew Wilcox
2018-08-21T13:35:54Z

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@infradead.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.

Shaohua added the div64 usage initially, adding him to the cc.

> There is macro DIV_ROUND_UP in kernel.h

Indeed there is.