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.

Link: https://lkml.kernel.org/r/20241025141724.17927-1-gourry@gourry.net
Fixes: 26aa2d199d6f ("mm/migrate: demote pages during reclaim")
Signed-off-by: Gregory Price <gourry@gourry.net>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Wei Xu <weixugc@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/migrate.c b/mm/migrate.c
index 7e520562d421a83d7a1246aa8b1071414d4b716a..fab84a7760889c4654e2a5a98de349caf23c482f 100644 (file)
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -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));