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 08:36:12 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
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 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.
>