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

Konstantin Khlebnikov
2018-08-21T05:11:50Z

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.

There is macro DIV_ROUND_UP in kernel.h