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

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

[Liu Shixin]

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@huawei.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

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

[Andrew Morton]

On Wed, 8 Sep 2021 09:56:06 +0800 Liu Shixin <liushixin2@huawei.com> wrote:

> 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().

checkpatch helpfully says

WARNING: data_race without comment
#40: FILE: mm/vmstat.c:1074:
+               blocks = data_race(zone->free_area[order].nr_free);


can we please add a comment ;)