[PATCH v3 3/8] mm/compaction: skip page block marked skip in isolate_migratepages_block

[Kemeng Shi]

Move migrate_pfn to page block end when block is marked skip to avoid
unnecessary scan retry of that block from upper caller.
For example, compact_zone may wrongly rescan skip page block with
finish_pageblock set as following:
1. cc->migrate point to the start of page block
2. compact_zone record last_migrated_pfn to cc->migrate
3. compact_zone->isolate_migratepages->isolate_migratepages_block tries to
scan the block. The low_pfn maybe moved forward to middle of block because
of free pages at beginning of block.
4. we find first lru page could be isolated but block was exclusive marked
skip.
5. abort isolate_migratepages_block and make cc->migrate_pfn point to
found lru page at middle of block.
6. compact_zone find cc->migrate_pfn and last_migrated_pfn are in the same
block and wrongly rescan the block with finish_pageblock set.

Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
 mm/compaction.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/compaction.c b/mm/compaction.c
index bdb03f38b7f7..9a2f0bd5394a 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1141,6 +1141,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 				skip_updated = true;
 				if (test_and_set_skip(cc, valid_page) &&
 				    !cc->finish_pageblock) {
+					low_pfn = end_pfn;
 					goto isolate_abort;
 				}
 			}
-- 
2.30.0