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

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Date:	Tue, 16 Aug 2016 15:46:31 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...nel.org>,
	Mel Gorman <mgorman@...hsingularity.net>,
	David Rientjes <rientjes@...gle.com>,
	Rik van Riel <riel@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 10/11] mm, compaction: require only min watermarks for
 non-costly orders

On Tue, Aug 16, 2016 at 08:36:12AM +0200, Vlastimil Babka wrote:
> 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.

Oops. I missed that. Thanks for clarifying it.

Thanks.