Commit a9aacbcc authored Feb 22, 2013 by Mel Gorman Committed by Linus Torvalds Feb 23, 2013

mm: compaction: do not accidentally skip pageblocks in the migrate scanner

Compaction uses the ALIGN macro incorrectly with the migrate scanner by
adding pageblock_nr_pages to a PFN. It happened to work when initially
implemented as the starting PFN was also aligned but with caching
restarts and isolating in smaller chunks this is no longer always true.

The impact is that the migrate scanner scans outside its current
pageblock. As pfn_valid() is still checked properly it does not cause
any failure and the impact of the bug is that in some cases it will scan
more than necessary when it crosses a page boundary but by no more than
COMPACT_CLUSTER_MAX. It is highly unlikely this is even measurable but
it's still wrong so this patch addresses the problem.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 62b726c1

mm/compaction.c

+2 −3

Original line number	Diff line number	Diff line
		@@ -611,8 +611,7 @@ check_compact_cluster:
		continue;

		next_pageblock:
		low_pfn += pageblock_nr_pages;
		low_pfn = ALIGN(low_pfn, pageblock_nr_pages) - 1;
		low_pfn = ALIGN(low_pfn + 1, pageblock_nr_pages) - 1;
		last_pageblock_nr = pageblock_nr;
		}

		@@ -795,7 +794,7 @@ static isolate_migrate_t isolate_migratepages(struct zone *zone,
		low_pfn = max(cc->migrate_pfn, zone->zone_start_pfn);

		/* Only scan within a pageblock boundary */
		end_pfn = ALIGN(low_pfn + pageblock_nr_pages, pageblock_nr_pages);
		end_pfn = ALIGN(low_pfn + 1, pageblock_nr_pages);

		/* Do not cross the free scanner or scan within a memory hole */
		if (end_pfn > cc->free_pfn \|\| !pfn_valid(low_pfn)) {

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