+ vmscanmigrate-fix-double-decrement-on-node-stats-when-demoting-pages.patch added to mm-hotfixes-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: vmscan,migrate: fix double-decrement on node stats when demoting pages
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     vmscanmigrate-fix-double-decrement-on-node-stats-when-demoting-pages.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/vmscanmigrate-fix-double-decrement-on-node-stats-when-demoting-pages.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Gregory Price <gourry@gourry.net>
Subject: vmscan,migrate: fix double-decrement on node stats when demoting pages
Date: Fri, 25 Oct 2024 10:17:24 -0400

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.

Link: https://lkml.kernel.org/r/20241025141724.17927-1-gourry@gourry.net
Fixes: 26aa2d199d6f2 ("mm/migrate: demote pages during reclaim")
Signed-off-by: Gregory Price <gourry@gourry.net>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Wei Xu <weixugc@google.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

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

--- a/mm/migrate.c~vmscanmigrate-fix-double-decrement-on-node-stats-when-demoting-pages
+++ a/mm/migrate.c
@@ -1178,7 +1178,7 @@ static void migrate_folio_done(struct fo
 	 * 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));
 
_

Patches currently in -mm which might be from gourry@gourry.net are

vmscanmigrate-fix-double-decrement-on-node-stats-when-demoting-pages.patch