[PATCH mmotm 2/3] mm, compaction: fix crash in get_pfnblock_flags_mask() from isolate_freepages():

[Vlastimil Babka <vbabka@suse.cz>]

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[].

Fixes: mmotm mm-compaction-wrap-calculating-first-and-last-pfn-of-pageblock.patch
Reported-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/compaction.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 759c3ac73ced..6a49d1b35515 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -999,7 +999,7 @@ 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

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