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

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Date:	Tue, 26 Aug 2008 10:29:29 +0100
From:	Andy Whitcroft <apw@...dowen.org>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Adam Litke <agl@...ibm.com>, 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>, agl <agl@...ux.vnet.ibm.com>
Subject: Re: [BUG] [PATCH v2] Make setup_zone_migrate_reserve() aware of
	overlapping nodes

On Thu, Aug 21, 2008 at 12:33:39PM +0100, Mel Gorman wrote:
> 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;

This patch looks sane.  I do note that we have a config option to tell
us whether we have any possibility of overlapping nodes, and we have an
early version of a check for this early_pfn_in_nid() in mm.h.  You might
consider having a non-early variant of this which could be optimised
away for those arches which do not have CONFIG_NODES_SPAN_OTHER_NODES.

In 'unearlifying' this to pfn_in_nid() I think we have a small naming
issue with these function as they are only valid for use with pfns within
an existing node.  They should probabally both be *pfn_in_nid_within()
or something in line with pfn_valid_within().

-apw
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