linux-kernel - mm/compact: why use low watermark to determine whether compact is finished instead of use high watermark?

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Date:	Fri, 22 Jul 2016 16:56:52 +0800
From:	Xishi Qiu <qiuxishi@...wei.com>
To:	"mel@....ul.ie" <mel@....ul.ie>, Vlastimil Babka <vbabka@...e.cz>,
	"David Rientjes" <rientjes@...gle.com>,
	Joonsoo Kim <js1304@...il.com>,
	Mel Gorman <mgorman@...hsingularity.net>
CC:	Linux MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>
Subject: mm/compact: why use low watermark to determine whether compact is
 finished instead of use high watermark?

Hi,

I find all the watermarks in mm/compaction.c are low_wmark_pages(),
so why not use high watermark to determine whether compact is finished?

e.g. 
__alloc_pages_nodemask()
	get_page_from_freelist()
	this is fast path, use use low_wmark_pages() in __zone_watermark_ok()

	__alloc_pages_slowpath()
	this is slow path, usually use min_wmark_pages()

kswapd
	balance_pgdat()
	use high_wmark_pages() to determine whether zone is balanced

Thanks,
Xishi Qiu