Re: [PATCH v6 08/11] mm, compaction: create compact_gap wrapper

[Vlastimil Babka]

On 08/16/2016 08:15 AM, Joonsoo Kim wrote:
> On Wed, Aug 10, 2016 at 11:12:23AM +0200, Vlastimil Babka wrote:
> > --- a/include/linux/compaction.h
> > +++ b/include/linux/compaction.h
> > @@ -58,6 +58,22 @@ enum compact_result {
> >
> >  struct alloc_context; /* in mm/internal.h */
> >
> > +/*
> > + * Number of free order-0 pages that should be available above given watermark
> > + * to make sure compaction has reasonable chance of not running out of free
> > + * pages that it needs to isolate as migration target during its work.
> > + */
> > +static inline unsigned long compact_gap(unsigned int order)
> > +{
> > +	/*
> > +	 * Although all the isolations for migration are temporary, compaction
> > +	 * may have up to 1 << order pages on its list and then try to split
> > +	 * an (order - 1) free page. At that point, a gap of 1 << order might
> > +	 * not be enough, so it's safer to require twice that amount.
> > +	 */
> > +	return 2UL << order;
> > +}
>
> I agree with this wrapper function but there is a question.
>
> Could you elaborate more on this code comment? Freescanner could keep
> COMPACT_CLUSTER_MAX freepages on the list. It's not associated with
> requested order at least for now. Why compact_gap is 2UL << order in
> this case?

It's true that for high enough order, COMPACT_CLUSTER_MAX might be more 
limiting than 1 << order. But then it also helps to have more free pages 
for probability of compaction success, so I don't think it's worth 
complicating the compact_gap() formula.

> Thanks.