linux-kernel - Re: [PATCH 07/11] Memory compaction core

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Date:	Wed, 24 Mar 2010 13:33:47 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 07/11] Memory compaction core

On Tue, 23 Mar 2010 12:25:42 +0000
Mel Gorman <mel@....ul.ie> wrote:

> This patch is the core of a mechanism which compacts memory in a zone by
> relocating movable pages towards the end of the zone.
> 
> A single compaction run involves a migration scanner and a free scanner.
> Both scanners operate on pageblock-sized areas in the zone. The migration
> scanner starts at the bottom of the zone and searches for all movable pages
> within each area, isolating them onto a private list called migratelist.
> The free scanner starts at the top of the zone and searches for suitable
> areas and consumes the free pages within making them available for the
> migration scanner. The pages isolated for migration are then migrated to
> the newly isolated free pages.

General comment: it looks like there are some codepaths which could
hold zone->lock for a long time.  It's unclear that they're all
constrained by COMPACT_CLUSTER_MAX. Is there a a latency issue here?

>
> ...
>
> +static struct page *compaction_alloc(struct page *migratepage,
> +					unsigned long data,
> +					int **result)
> +{
> +	struct compact_control *cc = (struct compact_control *)data;
> +	struct page *freepage;
> +
> +	VM_BUG_ON(cc == NULL);

It's a bit strange to test this when we're about to oops anyway.  The
oops will tell us the same thing.

> +	/* Isolate free pages if necessary */
> +	if (list_empty(&cc->freepages)) {
> +		isolate_freepages(cc->zone, cc);
> +
> +		if (list_empty(&cc->freepages))
> +			return NULL;
> +	}
> +
> +	freepage = list_entry(cc->freepages.next, struct page, lru);
> +	list_del(&freepage->lru);
> +	cc->nr_freepages--;
> +
> +	return freepage;
> +}

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