linux-kernel - Re: [PATCH 3/3] compaction accouting fix

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Message-ID: <20110831113710.GC17512@redhat.com>
Date:	Wed, 31 Aug 2011 13:37:10 +0200
From:	Johannes Weiner <jweiner@...hat.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 3/3] compaction accouting fix

On Sun, Nov 13, 2011 at 01:37:43AM +0900, Minchan Kim wrote:
> I saw the following accouting of compaction during test of the series.
> 
> compact_blocks_moved 251
> compact_pages_moved 44
> 
> It's very awkward to me although it's possbile because it means we try to compact 251 blocks
> but it just migrated 44 pages. As further investigation, I found isolate_migratepages doesn't
> isolate any pages but it returns ISOLATE_SUCCESS and then, it just increases compact_blocks_moved
> but doesn't increased compact_pages_moved.
> 
> This patch makes accouting of compaction works only in case of success of isolation.
> 
> CC: Mel Gorman <mgorman@...e.de>
> CC: Johannes Weiner <jweiner@...hat.com>
> CC: Rik van Riel <riel@...hat.com>
> Signed-off-by: Minchan Kim <minchan.kim@...il.com>

Acked-by: Johannes Weiner <jweiner@...hat.com>

It's a teensy-bit awkward that isolate_migratepages() can return
success without actually isolating any new pages, just because there
are still some pages left from a previous run (cc->nr_migratepages is
maintained across isolation calls).

Maybe isolate_migratepages() should just return an error if compaction
should really be aborted and 0 otherwise, and have compact_zone()
always check for cc->nr_migratepages itself?

	if (isolate_migratepages(zone, cc) < 0) {
		ret = COMPACT_PARTIAL;
		goto out;
	}

	if (!cc->nr_migratepages)
		continue;

	...

Just a nit-pick, though.  If you don't agree, just leave it as is.
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