The patch titled Subject: vmscan,migrate: fix page count imbalance 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...

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 page count imbalance 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. 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 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: 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