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.

[lizhijian@fujitsu.com: introduce local nr_demoted to fix nr_reclaimed double counting]
Link: https://lkml.kernel.org/r/20250111015253.425693-1-lizhijian@fujitsu.com
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 5b2f12425b28ce2bcfac5f0e2e631b480200c997..c767d71c43d7d202244d1e4b4fdedcb75c39979d 100644 (file)
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1086,7 +1086,7 @@ 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;
@@ -1550,8 +1550,9 @@ keep:
        /* '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 */