linux-kernel - [PATCH] mm/vmstat: annotate data race for zone->free_area[order].nr

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Date:   Wed, 8 Sep 2021 09:56:06 +0800
From:   Liu Shixin <liushixin2@...wei.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        "Paul E . McKenney" <paulmck@...nel.org>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Liu Shixin <liushixin2@...wei.com>
Subject: [PATCH] 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()/frag_show_print()
is lockless. That's intentional and the stats are a rough estimate anyway.
Annotate them with data_race().

Signed-off-by: Liu Shixin <liushixin2@...wei.com>
---
 mm/vmstat.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/vmstat.c b/mm/vmstat.c
index 0885a34197b7..17d1f9ec4fd4 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1023,7 +1023,7 @@ static void fill_contig_page_info(struct zone *zone,
 		unsigned long blocks;
 
 		/* Count number of free blocks */
-		blocks = zone->free_area[order].nr_free;
+		blocks = data_race(zone->free_area[order].nr_free);
 		info->free_blocks_total += blocks;
 
 		/* Count free base pages */
@@ -1397,7 +1397,7 @@ 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);
+		seq_printf(m, "%6lu ", data_race(zone->free_area[order].nr_free));
 	seq_putc(m, '\n');
 }
 
-- 
2.18.0.huawei.25