[PATCH] mm/vmstat: annotate data race for per-cpu pageset fields

["Hui Zhu" <hui.zhu@linux.dev>]

From: Hui Zhu <zhuhui@kylinos.cn>

zoneinfo_show_print() reads pcp->count, pcp->high, pcp->batch,
pcp->high_min, pcp->high_max and the per-cpu stat_threshold while
holding only zone->lock, which does not synchronize these fields.
The writers are the page allocation and free fast paths under
pcp->lock, decay_pcp_high() which updates pcp->high without any
lock, pageset_update() which writes batch/high_min/high_max with
WRITE_ONCE(), and refresh_zone_stat_thresholds() which writes
stat_threshold locklessly.

The race is benign: the values are only printed to /proc/zoneinfo,
they are naturally aligned integers, and pageset_update() already
documents that users of batch/high_min/high_max must cope with the
fields changing asynchronously.  Annotate the reads with data_race(),
following commit af1c31acc853 ("mm/vmstat: annotate data race for
zone->free_area[order].nr_free").

Found by KCSAN testing on an older kernel; the same race still
exists on mainline.  No functional change intended.

BUG: KCSAN: data-race in zoneinfo_show_print+0x355/0x520 root/klinux/mm/vmstat.c:1774
race at unknown origin, with read to 0xffff8e1835410808 of 4 bytes by task 22653 on cpu 12:
zoneinfo_show_print+0x355/0x520 root/klinux/mm/vmstat.c:1774
walk_zones_in_node root/klinux/mm/vmstat.c:1496 [inline]
zoneinfo_show+0x41/0x70 root/klinux/mm/vmstat.c:1806
seq_read_iter+0x30c/0x970 root/klinux/fs/seq_file.c:230
proc_reg_read_iter+0x10c/0x170 root/klinux/fs/proc/inode.c:305
copy_splice_read+0x2a1/0x4e0 root/klinux/fs/splice.c:365
do_splice_read root/klinux/fs/splice.c:985 [inline]
do_splice_read+0x139/0x1a0 root/klinux/fs/splice.c:959
splice_direct_to_actor+0x16b/0x540 root/klinux/fs/splice.c:1089
do_splice_direct_actor root/klinux/fs/splice.c:1207 [inline]
do_splice_direct+0x10a/0x180 root/klinux/fs/splice.c:1233
do_sendfile+0x6ea/0x7e0 root/klinux/fs/read_write.c:1363
__do_sys_sendfile64 root/klinux/fs/read_write.c:1424 [inline]
__se_sys_sendfile64 root/klinux/fs/read_write.c:1410 [inline]
__x64_sys_sendfile64+0x117/0x130 root/klinux/fs/read_write.c:1410
x64_sys_call+0x1cc7/0x1ee0 root/klinux/./arch/x86/include/generated/asm/syscalls_64.h:41
do_syscall_x64 root/klinux/arch/x86/entry/common.c:46 [inline]
do_syscall_64+0x75/0x2c0 root/klinux/arch/x86/entry/common.c:76
entry_SYSCALL_64_after_hwframe+0x76/0xe0
value changed: 0x000001b2 -> 0x000001b1
Reported by Kernel Concurrency Sanitizer on:
CPU: 12 PID: 22653 Comm: syz-executor.12 Not tainted 6.6.140+ #672
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014

Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>
---
 mm/vmstat.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/mm/vmstat.c b/mm/vmstat.c
index cb57714539fb..a3e809c57f29 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1837,6 +1837,11 @@ static void zoneinfo_show_print(struct seq_file *m, pg_data_t *pgdat,
 		struct per_cpu_zonestat __maybe_unused *pzstats;
 
 		pcp = per_cpu_ptr(zone->per_cpu_pageset, i);
+		/*
+		 * Access to the per-cpu pageset fields is lockless as they
+		 * are used only for printing purposes. Use data_race to
+		 * avoid KCSAN warning.
+		 */
 		seq_printf(m,
 			   "\n    cpu: %i"
 			   "\n              count:    %i"
@@ -1845,15 +1850,15 @@ static void zoneinfo_show_print(struct seq_file *m, pg_data_t *pgdat,
 			   "\n              high_min: %i"
 			   "\n              high_max: %i",
 			   i,
-			   pcp->count,
-			   pcp->high,
-			   pcp->batch,
-			   pcp->high_min,
-			   pcp->high_max);
+			   data_race(pcp->count),
+			   data_race(pcp->high),
+			   data_race(pcp->batch),
+			   data_race(pcp->high_min),
+			   data_race(pcp->high_max));
 #ifdef CONFIG_SMP
 		pzstats = per_cpu_ptr(zone->per_cpu_zonestats, i);
 		seq_printf(m, "\n  vm stats threshold: %d",
-				pzstats->stat_threshold);
+				data_race(pzstats->stat_threshold));
 #endif
 	}
 	seq_printf(m,
-- 
2.53.0