linux-kernel - Re: [PATCH 01/11] mm: page_alloc: handle MIGRATE_ISOLATE in free_pcppages

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Date:	Mon, 12 Dec 2011 13:42:35 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Marek Szyprowski <m.szyprowski@...sung.com>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-media@...r.kernel.org, linux-mm@...ck.org,
	linaro-mm-sig@...ts.linaro.org,
	Michal Nazarewicz <mina86@...a86.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Russell King <linux@....linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Ankita Garg <ankita@...ibm.com>,
	Daniel Walker <dwalker@...eaurora.org>,
	Arnd Bergmann <arnd@...db.de>,
	Jesse Barker <jesse.barker@...aro.org>,
	Jonathan Corbet <corbet@....net>,
	Shariq Hasnain <shariq.hasnain@...aro.org>,
	Chunsang Jeong <chunsang.jeong@...aro.org>,
	Dave Hansen <dave@...ux.vnet.ibm.com>
Subject: Re: [PATCH 01/11] mm: page_alloc: handle MIGRATE_ISOLATE in
 free_pcppages_bulk()

On Fri, Nov 18, 2011 at 05:43:08PM +0100, Marek Szyprowski wrote:
> From: Michal Nazarewicz <mina86@...a86.com>
> 
> If page is on PCP list while pageblock it belongs to gets isolated,
> the page's private still holds the old migrate type.  This means
> that free_pcppages_bulk() will put the page on a freelist of the
> old migrate type instead of MIGRATE_ISOLATE.
> 
> This commit changes that by explicitly checking whether page's
> pageblock's migrate type is MIGRATE_ISOLATE and if it is, overwrites
> page's private data.
> 
> Signed-off-by: Michal Nazarewicz <mina86@...a86.com>
> Signed-off-by: Marek Szyprowski <m.szyprowski@...sung.com>
> ---
>  mm/page_alloc.c |   12 ++++++++++++
>  1 files changed, 12 insertions(+), 0 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 9dd443d..58d1a2e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -628,6 +628,18 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>  			page = list_entry(list->prev, struct page, lru);
>  			/* must delete as __free_one_page list manipulates */
>  			list_del(&page->lru);
> +
> +			/*
> +			 * When page is isolated in set_migratetype_isolate()
> +			 * function it's page_private is not changed since the
> +			 * function has no way of knowing if it can touch it.
> +			 * This means that when a page is on PCP list, it's
> +			 * page_private no longer matches the desired migrate
> +			 * type.
> +			 */
> +			if (get_pageblock_migratetype(page) == MIGRATE_ISOLATE)
> +				set_page_private(page, MIGRATE_ISOLATE);
> +

How much of a problem is this in practice? It's adding overhead to the
free path for what should be a very rare case which is undesirable. I
know we are already calling get_pageblock_migrate() when freeing
pages but it should be unnecessary to call it again. I'd go as far
to say that it would be preferable to drain the per-CPU lists after
you set pageblocks MIGRATE_ISOLATE. The IPIs also have overhead but it
will be incurred for the rare rather than the common case.

-- 
Mel Gorman
SUSE Labs
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