mm/vmscan: accumulate nr_demoted for accurate demotion statistics

In shrink_folio_list(), 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.

Link: https://lkml.kernel.org/r/20250110122133.423481-1-lizhijian@fujitsu.com
Fixes: f77f0c751478 ("mm,memcg: provide per-cgroup counters for NUMA balancing operations")
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Acked-by: Kaiyang Zhao <kaiyang2@cs.cmu.edu>
Tested-by: Donet Tom <donettom@linux.ibm.com>
Reviewed-by: Donet Tom <donettom@linux.ibm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 985102d2aa203a88a31bfbcb0eefc34eeb16ba3b..7fd84c7a99b05dfc63cabbb288efdad7a3fe3427 100644 (file)
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1522,7 +1522,7 @@ keep:
        /* 'folio_list' is always empty here */
 
        /* Migrate folios selected for demotion */
-       stat->nr_demoted = demote_folio_list(&demote_folios, pgdat);
+       stat->nr_demoted += demote_folio_list(&demote_folios, pgdat);
        nr_reclaimed += stat->nr_demoted;
        /* Folios that could not be demoted are still in @demote_folios */
        if (!list_empty(&demote_folios)) {