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>

diff --git a/mm/compaction.c b/mm/compaction.c
index c62bd063d766c7333ca0370444636a52ddc70802..0d0248db36d8a4b9bc62f6082cf4e42505d33890 100644 (file)
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -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)) {