[PATCH v2 2/8] mm/compaction: correct last_migrated_pfn update in compact_zone

[Kemeng Shi]

We record start pfn of last isolated page block with last_migrated_pfn. And
then:
1. We check if we mark the page block skip for exclusive access in
isolate_migratepages_block by test if next migrate pfn is still in last
isolated page block. If so, we will set finish_pageblock to do the rescan.
2. We check if a full cc->order block is scanned by test if last scan range
passes the cc->order block boundary. If so, we flush the pages were freed.

We treat cc->migrate_pfn before isolate_migratepages as the start pfn of
last isolated page range. However, we always align migrate_pfn to page block
or move to another page block in fast_find_migrateblock or in linearly scan
forward in isolate_migratepages before do page isolation in
isolate_migratepages_block.

Update last_migrated_pfn with pageblock_start_pfn(cc->migrate_pfn - 1)
after scan to correctly set start pfn of last isolated page range. To
avoid that:
1. Miss a rescan with finish_pageblock set as last_migrate_pfn does not
point to right pageblock and the migrate will not be in pageblock of
last_migrate_pfn as it should be.
2. Wrongly issue flush by test cc->order block boundary with wrong
last_migrate_pfn.

Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
---
 mm/compaction.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index a8cea916df9d..ec3a96b7afce 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -2487,7 +2487,8 @@ compact_zone(struct compact_control *cc, struct capture_control *capc)
 			goto check_drain;
 		case ISOLATE_SUCCESS:
 			update_cached = false;
-			last_migrated_pfn = iteration_start_pfn;
+			last_migrated_pfn = max(cc->zone->zone_start_pfn,
+				pageblock_start_pfn(cc->migrate_pfn - 1));
 		}
 
 		err = migrate_pages(&cc->migratepages, compaction_alloc,
-- 
2.30.0