Re: [BUG] [PATCH v2] Make setup_zone_migrate_reserve() aware of overlapping nodes

Andy Whitcroft
2008-08-26T09:29:29Z

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@us.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@csn.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