Re: [PATCH v6 10/11] mm, compaction: require only min watermarks for non-costly orders

[Vlastimil Babka]

On 08/16/2016 08:16 AM, Joonsoo Kim wrote:
> On Wed, Aug 10, 2016 at 11:12:25AM +0200, Vlastimil Babka wrote:
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 621e4211ce16..a5c0f914ec00 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -2492,7 +2492,7 @@ int __isolate_free_page(struct page *page, unsigned int order)
> >
> >  	if (!is_migrate_isolate(mt)) {
> >  		/* Obey watermarks as if the page was being allocated */
> > -		watermark = low_wmark_pages(zone) + (1 << order);
> > +		watermark = min_wmark_pages(zone) + (1UL << order);
>
> This '1 << order' also needs some comment. Why can't we use
> compact_gap() in this case?

This is just short-cutting the high-order watermark check to check only 
order-0, because we already know the high-order page exists.
We can't use compact_gap() as that's too high to use for a single 
allocation watermark, since we can be already holding some free pages on 
the list. So it would defeat the gap purpose.

> Thanks.