mm/vmstat: annotate data race for zone->free_area[order].nr_free

KCSAN reports a data-race on v5.10 which also exists on mainline:

  BUG: KCSAN: data-race in extfrag_for_order+0x33/0x2d0

  race at unknown origin, with read to 0xffff9ee9bfffab48 of 8 bytes by task 34 on cpu 1:
   extfrag_for_order+0x33/0x2d0
   kcompactd+0x5f0/0xce0
   kthread+0x1f9/0x220
   ret_from_fork+0x22/0x30

  Reported by Kernel Concurrency Sanitizer on:
  CPU: 1 PID: 34 Comm: kcompactd0 Not tainted 5.10.0+ #2
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014

Access to zone->free_area[order].nr_free in extfrag_for_order() and
frag_show_print() is lockless.  That's intentional and the stats are a
rough estimate anyway.  Annotate them with data_race().

[liushixin2@huawei.com: add comments]
  Link: https://lkml.kernel.org/r/20210918084655.2696522-1-liushixin2@huawei.com

Link: https://lkml.kernel.org/r/20210908015606.3999871-1-liushixin2@huawei.comSigned-off-by: Liu Shixin <liushixin2@huawei.com>
Cc: "Paul E . McKenney" <paulmck@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmstat.c

@@ -1070,8 +1070,13 @@ static void fill_contig_page_info(struct zone *zone,
 	for (order = 0; order < MAX_ORDER; order++) {
 		unsigned long blocks;
 
-		/* Count number of free blocks */
-		blocks = zone->free_area[order].nr_free;
+		/*
+		 * Count number of free blocks.
+		 *
+		 * Access to nr_free is lockless as nr_free is used only for
+		 * diagnostic purposes. Use data_race to avoid KCSAN warning.
+		 */
+		blocks = data_race(zone->free_area[order].nr_free);
 		info->free_blocks_total += blocks;
 
 		/* Count free base pages */
@@ -1446,7 +1451,11 @@ static void frag_show_print(struct seq_file *m, pg_data_t *pgdat,
 
 	seq_printf(m, "Node %d, zone %8s ", pgdat->node_id, zone->name);
 	for (order = 0; order < MAX_ORDER; ++order)
-		seq_printf(m, "%6lu ", zone->free_area[order].nr_free);
+		/*
+		 * Access to nr_free is lockless as nr_free is used only for
+		 * printing purposes. Use data_race to avoid KCSAN warning.
+		 */
+		seq_printf(m, "%6lu ", data_race(zone->free_area[order].nr_free));
 	seq_putc(m, '\n');
 }