mm: add node_reclaim successes to VM event counters

/proc/vmstat currently shows the number of node_reclaim() failures when
vm.zone_reclaim_mode is set appropriately.  It would be convenient to have
the number of successes right next to zone_reclaim_failed (similar to
compaction and migration).

While just a trivially addition to the vmstat file.  It was helpful during
benchmarking to not have to probe node_reclaim() to observe the
success/failure ratio.

Link: https://lkml.kernel.org/r/20240722171316.7517-1-mcassell411@gmail.com
Signed-off-by: Matthew Cassell <mcassell411@gmail.com>
Cc: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Li Zhijian <lizhijian@fujitsu.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Matthew Cassell

include/linux/vm_event_item.h

@@ -50,6 +50,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
 		PGSTEAL_ANON,
 		PGSTEAL_FILE,
 #ifdef CONFIG_NUMA
+		PGSCAN_ZONE_RECLAIM_SUCCESS,
 		PGSCAN_ZONE_RECLAIM_FAILED,
 #endif
 		PGINODESTEAL, SLABS_SCANNED, KSWAPD_INODESTEAL,

mm/vmscan.c

@@ -7539,7 +7539,9 @@ int node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned int order)
 	ret = __node_reclaim(pgdat, gfp_mask, order);
 	clear_bit(PGDAT_RECLAIM_LOCKED, &pgdat->flags);
 
-	if (!ret)
+	if (ret)
+		count_vm_event(PGSCAN_ZONE_RECLAIM_SUCCESS);
+	else
 		count_vm_event(PGSCAN_ZONE_RECLAIM_FAILED);
 
 	return ret;

mm/vmstat.c

@@ -1314,6 +1314,7 @@ const char * const vmstat_text[] = {
 	"pgsteal_file",
 
 #ifdef CONFIG_NUMA
+	"zone_reclaim_success",
 	"zone_reclaim_failed",
 #endif
 	"pginodesteal",