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.

---
 mm/compaction.c |    5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

index 9eef558..cad98f6 100644
@@ -630,8 +630,7 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
 		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;
 	}
 

@@ -802,7 +801,7 @@ 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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