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Date: Tue, 25 Feb 2020 22:58:27 -0500
From: Qian Cai <cai@....pw>
To: akpm@...ux-foundation.org
Cc: elver@...gle.com, willy@...radead.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, Qian Cai <cai@....pw>
Subject: [PATCH v2] mm/vmscan: fix data races at kswapd_classzone_idx
pgdat->kswapd_classzone_idx could be accessed concurrently in
wakeup_kswapd(). Plain writes and reads without any lock protection
result in data races. Fix them by adding a pair of READ|WRITE_ONCE() as
well as saving a branch (compilers might well optimize the original code
in an unintentional way anyway). While at it, also take care of
pgdat->kswapd_order and non-kswapd threads in allow_direct_reclaim().
The data races were reported by KCSAN,
BUG: KCSAN: data-race in wakeup_kswapd / wakeup_kswapd
write to 0xffff9f427ffff2dc of 4 bytes by task 7454 on cpu 13:
wakeup_kswapd+0xf1/0x400
wakeup_kswapd at mm/vmscan.c:3967
wake_all_kswapds+0x59/0xc0
wake_all_kswapds at mm/page_alloc.c:4241
__alloc_pages_slowpath+0xdcc/0x1290
__alloc_pages_slowpath at mm/page_alloc.c:4512
__alloc_pages_nodemask+0x3bb/0x450
alloc_pages_vma+0x8a/0x2c0
do_anonymous_page+0x16e/0x6f0
__handle_mm_fault+0xcd5/0xd40
handle_mm_fault+0xfc/0x2f0
do_page_fault+0x263/0x6f9
page_fault+0x34/0x40
1 lock held by mtest01/7454:
#0: ffff9f425afe8808 (&mm->mmap_sem#2){++++}, at:
do_page_fault+0x143/0x6f9
do_user_addr_fault at arch/x86/mm/fault.c:1405
(inlined by) do_page_fault at arch/x86/mm/fault.c:1539
irq event stamp: 6944085
count_memcg_event_mm+0x1a6/0x270
count_memcg_event_mm+0x119/0x270
__do_softirq+0x34c/0x57c
irq_exit+0xa2/0xc0
read to 0xffff9f427ffff2dc of 4 bytes by task 7472 on cpu 38:
wakeup_kswapd+0xc8/0x400
wake_all_kswapds+0x59/0xc0
__alloc_pages_slowpath+0xdcc/0x1290
__alloc_pages_nodemask+0x3bb/0x450
alloc_pages_vma+0x8a/0x2c0
do_anonymous_page+0x16e/0x6f0
__handle_mm_fault+0xcd5/0xd40
handle_mm_fault+0xfc/0x2f0
do_page_fault+0x263/0x6f9
page_fault+0x34/0x40
1 lock held by mtest01/7472:
#0: ffff9f425a9ac148 (&mm->mmap_sem#2){++++}, at:
do_page_fault+0x143/0x6f9
irq event stamp: 6793561
count_memcg_event_mm+0x1a6/0x270
count_memcg_event_mm+0x119/0x270
__do_softirq+0x34c/0x57c
irq_exit+0xa2/0xc0
Signed-off-by: Qian Cai <cai@....pw>
---
v2: use a temp variable and take care of kswapd_order per Matthew.
take care of allow_direct_reclaim() as well.
mm/vmscan.c | 20 ++++++++++++--------
1 file changed, 12 insertions(+), 8 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 876370565455..e61cc71b8915 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3136,8 +3136,9 @@ static bool allow_direct_reclaim(pg_data_t *pgdat)
/* kswapd must be awake if processes are being throttled */
if (!wmark_ok && waitqueue_active(&pgdat->kswapd_wait)) {
- pgdat->kswapd_classzone_idx = min(pgdat->kswapd_classzone_idx,
- (enum zone_type)ZONE_NORMAL);
+ if (READ_ONCE(pgdat->kswapd_classzone_idx) > ZONE_NORMAL)
+ WRITE_ONCE(pgdat->kswapd_classzone_idx, ZONE_NORMAL);
+
wake_up_interruptible(&pgdat->kswapd_wait);
}
@@ -3953,20 +3954,23 @@ void wakeup_kswapd(struct zone *zone, gfp_t gfp_flags, int order,
enum zone_type classzone_idx)
{
pg_data_t *pgdat;
+ enum zone_type curr_idx;
if (!managed_zone(zone))
return;
if (!cpuset_zone_allowed(zone, gfp_flags))
return;
+
pgdat = zone->zone_pgdat;
+ curr_idx = READ_ONCE(pgdat->kswapd_classzone_idx);
+
+ if (curr_idx == MAX_NR_ZONES || curr_idx < classzone_idx)
+ WRITE_ONCE(pgdat->kswapd_classzone_idx, classzone_idx);
+
+ if (READ_ONCE(pgdat->kswapd_order) < order)
+ WRITE_ONCE(pgdat->kswapd_order, order);
- if (pgdat->kswapd_classzone_idx == MAX_NR_ZONES)
- pgdat->kswapd_classzone_idx = classzone_idx;
- else
- pgdat->kswapd_classzone_idx = max(pgdat->kswapd_classzone_idx,
- classzone_idx);
- pgdat->kswapd_order = max(pgdat->kswapd_order, order);
if (!waitqueue_active(&pgdat->kswapd_wait))
return;
--
2.21.0 (Apple Git-122.2)
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