linux-kernel - Re: [PATCH] mm: compaction: Fix return value of capture_free

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Date:	Mon, 26 Nov 2012 07:06:53 -0800
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Mel Gorman <mgorman@...e.de>
CC:	akpm@...l.org, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: compaction: Fix return value of capture_free_page

On 11/26/2012 03:23 AM, Mel Gorman wrote:
> On Wed, Nov 21, 2012 at 02:21:51PM -0500, Dave Hansen wrote:
>>
>> This needs to make it in before 3.7 is released.
>>
> 
> This is also required. Dave, can you double check? The surprise is that
> this does not blow up very obviously.
...
> @@ -1422,7 +1422,7 @@ int capture_free_page(struct page *page, int alloc_order, int migratetype)
>  		}
>  	}
> 
> -	return 1UL << order;
> +	return 1UL << alloc_order;
>  }

compact_capture_page() only looks at the boolean return value out of
capture_free_page(), so it wouldn't notice.  split_free_page() does.
But, when it calls capture_free_page(), order==alloc_order, so it
wouldn't make a difference.  So, there's probably no actual bug here,
but it's certainly a wrong return value.

We should probably also fix the set_pageblock_migratetype() loop in
there while we're at it.  I think it's potentially trampling on the
migration type of pages currently in the allocator.  I _think_ that
completes the list of things that need to get audited in there.

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