[PATCH] mm, compaction: fix crash in get_pfnblock_flags_mask() from isolate_freepages():

[Vlastimil Babka]

In isolate_freepages(), low_pfn was mistakenly initialized to
pageblock_start_pfn() instead of pageblock_end_pfn(), creating a possible
underflow, as described by Hugh:

   There's a case when that "block_start_pfn -= pageblock_nr_pages" loop can
   pass through 0 and end up trying to access a pageblock before the start of
   the mem_map[].

Reported-by: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 mm/compaction.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 315e5d57e7e9..67f886ecd773 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1012,7 +1012,7 @@ static void isolate_freepages(struct compact_control *cc)
 	block_start_pfn = pageblock_start_pfn(cc->free_pfn);
 	block_end_pfn = min(block_start_pfn + pageblock_nr_pages,
 						zone_end_pfn(zone));
-	low_pfn = pageblock_start_pfn(cc->migrate_pfn);
+	low_pfn = pageblock_end_pfn(cc->migrate_pfn);
 
 	/*
 	 * Isolate free pages until enough are available to migrate the
-- 
2.8.1