linux-kernel - Re: [PATCH] mm: compaction: Abort compaction if too many pages are isolated and caller is asynchronous

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Date:	Mon, 6 Jun 2011 11:39:24 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Minchan Kim <minchan.kim@...il.com>, akpm@...ux-foundation.org,
	Ury Stankevich <urykhy@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, stable@...nel.org
Subject: Re: [PATCH] mm: compaction: Abort compaction if too many pages are
 isolated and caller is asynchronous

On Fri, Jun 03, 2011 at 05:45:54PM +0200, Andrea Arcangeli wrote:
> On Fri, Jun 03, 2011 at 03:49:41PM +0100, Mel Gorman wrote:
> > Right idea of the wrong zone being accounted for but wrong place. I
> > think the following patch should fix the problem;
> 
> Looks good thanks.
> 
> I also found this bug during my debugging that made NR_SHMEM underflow.
> 
> ===
> Subject: migrate: don't account swapcache as shmem
> 
> From: Andrea Arcangeli <aarcange@...hat.com>
> 
> swapcache will reach the below code path in migrate_page_move_mapping,
> and swapcache is accounted as NR_FILE_PAGES but it's not accounted as
> NR_SHMEM.
> 
> Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>

Well spotted.

Acked-by: Mel Gorman <mgorman@...e.de>

Minor nit. swapper_space is rarely referred to outside of the swap
code. Might it be more readable to use

	/*
	 * swapcache is accounted as NR_FILE_PAGES but it is not
	 * accounted as NR_SHMEM
	 *
	if (PageSwapBacked(page) && !PageSwapCache(page))

?

> ---
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index e4a5c91..2597a27 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -288,7 +288,7 @@ static int migrate_page_move_mapping(struct address_space *mapping,
>  	 */
>  	__dec_zone_page_state(page, NR_FILE_PAGES);
>  	__inc_zone_page_state(newpage, NR_FILE_PAGES);
> -	if (PageSwapBacked(page)) {
> +	if (mapping != &swapper_space && PageSwapBacked(page)) {
>  		__dec_zone_page_state(page, NR_SHMEM);
>  		__inc_zone_page_state(newpage, NR_SHMEM);
>  	}
> 
> 

-- 
Mel Gorman
SUSE Labs
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