When numa balancing is enabled with demotion, vmscan will call
migrate_pages when shrinking LRUs. Successful demotions will
cause node vmstat numbers to double-decrement, leading to an
imbalanced page count. The result is dmesg output like such:
$ cat /proc/sys/vm/stat_refresh
[77383.088417] vmstat_refresh: nr_isolated_anon -103212
[77383.088417] vmstat_refresh: nr_isolated_file -899642
This negative value may impact compaction and reclaim throttling.
The double-decrement occurs in the migrate_pages path:
caller to shrink_folio_list decrements the count
shrink_folio_list
demote_folio_list
migrate_pages
migrate_pages_batch
migrate_folio_move
migrate_folio_done
mod_node_page_state(-ve) <- second decrement
This path happens for SUCCESSFUL migrations, not failures. Typically
callers to migrate_pages are required to handle putback/accounting for
failures, but this is already handled in the shrink code.
When accounting for migrations, instead do not decrement the count
when the migration reason is MR_DEMOTION. As of v6.11, this demotion
logic is the only source of MR_DEMOTION.
Signed-off-by: Gregory Price <gourry@xxxxxxxxxx>
Fixes: 26aa2d199d6f2 ("mm/migrate: demote pages during reclaim")
Cc: stable@xxxxxxxxxxxxxxx
---
mm/migrate.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/migrate.c b/mm/migrate.c
index 923ea80ba744..e3aac274cf16 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1099,7 +1099,7 @@ static void migrate_folio_done(struct folio *src,
* not accounted to NR_ISOLATED_*. They can be recognized
* as __folio_test_movable
*/
- if (likely(!__folio_test_movable(src)))
+ if (likely(!__folio_test_movable(src)) && reason != MR_DEMOTION)
mod_node_page_state(folio_pgdat(src), NR_ISOLATED_ANON +
folio_is_file_lru(src), -folio_nr_pages(src));
--
2.43.0
