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Subject: Re: [PATCH v6 10/11] mm, compaction: require only min watermarks for
 non-costly orders
References: <20160810091226.6709-1-vbabka@suse.cz>
 <20160810091226.6709-11-vbabka@suse.cz>
 <20160816061636.GF17448@js1304-P5Q-DELUXE>
From: Vlastimil Babka <vbabka@suse.cz>
Message-ID: <484d17e5-7294-4724-f5f9-0a15167d47ee@suse.cz>
Date: Tue, 16 Aug 2016 08:36:12 +0200
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Sender: owner-linux-mm@kvack.org
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To: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Michal Hocko <mhocko@kernel.org>, Mel Gorman <mgorman@techsingularity.net>, David Rientjes <rientjes@google.com>, Rik van Riel <riel@redhat.com>, linux-mm@kvack.org, linux-kernel@vger.kernel.org

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.
>

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