FAILED: Patch "vmscan,migrate: fix page count imbalance on node stats when demoting pages" failed to apply to v5.15-stable tree

The patch below does not apply to the v5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 35e41024c4c2b02ef8207f61b9004f6956cf037b Mon Sep 17 00:00:00 2001
From: Gregory Price <[email protected]>
Date: Fri, 25 Oct 2024 10:17:24 -0400
Subject: [PATCH] vmscan,migrate: fix page count imbalance on node stats when
demoting pages

When numa balancing is enabled with demotion, vmscan will call migrate_pages when shrinking LRUs. migrate_pages will decrement the the node's isolated page count, leading to an imbalanced count when invoked from (MG)LRU code.

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 following path produces the decrement:

shrink_folio_list
demote_folio_list
migrate_pages
migrate_pages_batch
migrate_folio_move
migrate_folio_done
mod_node_page_state(-ve) <- 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.

---
 mm/migrate.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 7e520562d421a..fab84a7760889 100644
@@ -1178,7 +1178,7 @@ 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