linux-kernel - Re: [BUG] [PATCH v2] Make setup_zone_migrate

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Message-ID: <20080821113338.GA29950@csn.ul.ie>
Date:	Thu, 21 Aug 2008 12:33:39 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Adam Litke <agl@...ibm.com>
Cc:	Dave Hansen <dave@...ux.vnet.ibm.com>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	nacc <nacc@...ux.vnet.ibm.com>, apw <apw@...dowen.org>,
	agl <agl@...ux.vnet.ibm.com>
Subject: Re: [BUG] [PATCH v2] Make setup_zone_migrate_reserve() aware of overlapping nodes

On (20/08/08 14:55), Adam Litke didst pronounce:
>     Changes since V1
>      - Fix build for !NUMA
>      - Add VM_BUG_ON() to catch this problem at the source
>     
>     I have gotten to the root cause of the hugetlb badness I reported back on
>     August 15th.  My system has the following memory topology (note the
>     overlapping node):
>     
>             Node 0 Memory: 0x8000000-0x44000000
>             Node 1 Memory: 0x0-0x8000000 0x44000000-0x80000000
>     
>     setup_zone_migrate_reserve() scans the address range 0x0-0x8000000 looking
>     for a pageblock to move onto the MIGRATE_RESERVE list.  Finding no
>     candidates, it happily continues the scan into 0x8000000-0x44000000.  When
>     a pageblock is found, the pages are moved to the MIGRATE_RESERVE list on
>     the wrong zone.  Oops.
>     
>     (Andrew: once the proper fix is agreed upon, this should also be a
>     candidate for -stable.)
>     
>     setup_zone_migrate_reserve() should skip pageblocks in overlapping nodes.
>     
>     Signed-off-by: Adam Litke <agl@...ibm.com>
> 

zone_to_nid(zone) is called every time in the loop even though it will never
change. This is less than optimal but setup_zone_migrate_reserve() is only
called during init and when min_free_kbytes is adjusted so it's not worth
worrying about. Otherwise it looks good.

Acked-by: Mel Gorman <mel@....ul.ie>

> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index af982f7..feb7916 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -694,6 +694,9 @@ static int move_freepages(struct zone *zone,
>  #endif
>  
>  	for (page = start_page; page <= end_page;) {
> +		/* Make sure we are not inadvertently changing nodes */
> +		VM_BUG_ON(page_to_nid(page) != zone_to_nid(zone));
> +
>  		if (!pfn_valid_within(page_to_pfn(page))) {
>  			page++;
>  			continue;
> @@ -2516,6 +2519,10 @@ static void setup_zone_migrate_reserve(struct zone *zone)
>  			continue;
>  		page = pfn_to_page(pfn);
>  
> +		/* Watch out for overlapping nodes */
> +		if (page_to_nid(page) != zone_to_nid(zone))
> +			continue;
> +
>  		/* Blocks with reserved pages will never free, skip them. */
>  		if (PageReserved(page))
>  			continue;
> 
> -- 
> Adam Litke - (agl at us.ibm.com)
> IBM Linux Technology Center
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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