[PATCH v2] mm/vmscan: Accumulate nr_demoted for accurate demotion statistics

[Li Zhijian <lizhijian@fujitsu.com>]

In the shrink_folio_list() function, demote_folio_list() can be called
2 times. Currently stat->nr_demoted will only store the last nr_demoted(
the later nr_demoted is always zero, the former nr_demoted will get lost),
as a result number of demoted pages is not accurate.

Accumulate the nr_demoted count across multiple calls to
demote_folio_list(), ensuring accurate reporting of demotion statistics.

Fixes: f77f0c751478 ("mm,memcg: provide per-cgroup counters for NUMA balancing operations")
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
---

Cc: Kaiyang Zhao <kaiyang2@cs.cmu.edu>
Cc: Andrew Morton <akpm@linux-foundation.org>

Kaiyang, I removed you ACKED tag for V1, please review again
Andrew, Please drop the v1 from the mm-hotfixs-unstable if V2 is
accepted.

V2:
  Introudce a local nr_demoted to fix it's counted 2 times by nr_reclaimed wrongly.
---
 mm/vmscan.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 9a859b7d18d7..7f0624b98aa8 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1053,7 +1053,7 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 	struct folio_batch free_folios;
 	LIST_HEAD(ret_folios);
 	LIST_HEAD(demote_folios);
-	unsigned int nr_reclaimed = 0;
+	unsigned int nr_reclaimed = 0, nr_demoted = 0;
 	unsigned int pgactivate = 0;
 	bool do_demote_pass;
 	struct swap_iocb *plug = NULL;
@@ -1522,8 +1522,9 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 	/* 'folio_list' is always empty here */
 
 	/* Migrate folios selected for demotion */
-	stat->nr_demoted = demote_folio_list(&demote_folios, pgdat);
-	nr_reclaimed += stat->nr_demoted;
+	nr_demoted = demote_folio_list(&demote_folios, pgdat);
+	nr_reclaimed += nr_demoted;
+	stat->nr_demoted += nr_demoted;
 	/* Folios that could not be demoted are still in @demote_folios */
 	if (!list_empty(&demote_folios)) {
 		/* Folios which weren't demoted go back on @folio_list */
-- 
2.44.0