[RFC PATCH] mm/damon: adjust isolated pages stat for DAMOS_MIGRATE_{HOT,COLD}

[SJ Park <sj@kernel.org>]

Callers of migrate_pages() should adjust NR_MIGRATED_{ANON,FILE} for
isolations and putback of the folios.  That for migration succeeded
folios is done by migrate_pages(), in migrate_folio_done().  That for
MR_DEMOTION reason is an exception though.

DAMOS_MIGRATE_{HOT,COLD} call migrate_pages() but mistakenly not doing
the stat adjustment.  As a result, use of DAMOS_MIGRATE_{HOT,COLD} could
corrupt the stat.  It could confuse too_many_isolated(), make compaction
and reclaim to behave in unexpected ways.  The stat corruption can be
reproduced and confirmed using DAMON user-space tool [1] on NUMA
systems, like below.

    $ numactl --hardware
    available: 2 nodes (0-1)
    [...]
    $ sudo ./damo start --damos_action migrate_hot 1
    $ sudo cat /proc/sys/vm/stat_refresh
    $ sudo dmesg
    [...]
    [   80.215554] vmstat_refresh: nr_isolated_anon -5578
    [   80.216842] vmstat_refresh: nr_isolated_file -34400

This issue was discovered [2] by Sashiko.

[1] https://github.com/damonitor/damo
[2] https://lore.kernel.org/20260726164356.87940-1-sj@kernel.org

Fixes: b51820ebea65 ("mm/damon/paddr: introduce DAMOS_MIGRATE_COLD action for demotion")
Cc: <stable@vger.kernel.org> # 6.11.x
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/ops-common.c | 2 ++
 mm/damon/paddr.c      | 2 ++
 mm/damon/vaddr.c      | 3 ++-
 3 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/damon/ops-common.c b/mm/damon/ops-common.c
index e59f77eca83b2..9809fa12b09a8 100644
--- a/mm/damon/ops-common.c
+++ b/mm/damon/ops-common.c
@@ -374,6 +374,8 @@ static unsigned int damon_migrate_folio_list(struct list_head *folio_list,
 	while (!list_empty(folio_list)) {
 		folio = lru_to_folio(folio_list);
 		list_del(&folio->lru);
+		node_stat_sub_folio(folio, NR_ISOLATED_ANON +
+				folio_is_file_lru(folio));
 		folio_putback_lru(folio);
 	}
 
diff --git a/mm/damon/paddr.c b/mm/damon/paddr.c
index 5a6a78054784a..5c6c3a597fd0b 100644
--- a/mm/damon/paddr.c
+++ b/mm/damon/paddr.c
@@ -346,6 +346,8 @@ static unsigned long damon_pa_migrate(struct damon_region *r,
 
 		if (!folio_isolate_lru(folio))
 			goto put_folio;
+		node_stat_add_folio(folio, NR_ISOLATED_ANON +
+				folio_is_file_lru(folio));
 		list_add(&folio->lru, &folio_list);
 put_folio:
 		addr += folio_size(folio);
diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
index 4976775fe2762..0648400b2d65b 100644
--- a/mm/damon/vaddr.c
+++ b/mm/damon/vaddr.c
@@ -627,7 +627,8 @@ static void damos_va_migrate_dests_add(struct folio *folio,
 isolate:
 	if (!folio_isolate_lru(folio))
 		return;
-
+	node_stat_add_folio(folio, NR_ISOLATED_ANON +
+			folio_is_file_lru(folio));
 	list_add(&folio->lru, &migration_lists[i]);
 }
 

base-commit: 1e15fbef3a3c465a6a2db0f38e3980d75420bd6d
-- 
2.47.3