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Date: Tue, 16 Aug 2016 15:46:31 +0900
From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
To: Vlastimil Babka <vbabka@suse.cz>
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
Subject: Re: [PATCH v6 10/11] mm, compaction: require only min watermarks for
 non-costly orders
Message-ID: <20160816064630.GH17448@js1304-P5Q-DELUXE>
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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.