From: Liu Shixin <liushixin2@xxxxxxxxxx> Subject: 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(). [liushixin2@xxxxxxxxxx: add comments] Link: https://lkml.kernel.org/r/20210918084655.2696522-1-liushixin2@xxxxxxxxxx Link: https://lkml.kernel.org/r/20210908015606.3999871-1-liushixin2@xxxxxxxxxx Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx> Cc: "Paul E . McKenney" <paulmck@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/vmstat.c | 15 ++++++++++++--- 1 file changed, 12 insertions(+), 3 deletions(-) --- a/mm/vmstat.c~mm-vmstat-annotate-data-race-for-zone-free_areanr_free +++ a/mm/vmstat.c @@ -1070,8 +1070,13 @@ static void fill_contig_page_info(struct 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_f 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'); } _
