[PATCH] mm: memcontrol: fix data race in mem_cgroup_select_victim_node

[Shakeel Butt]

Syzbot reported the following bug:

BUG: KCSAN: data-race in mem_cgroup_select_victim_node / mem_cgroup_select_victim_node

write to 0xffff88809fade9b0 of 4 bytes by task 8603 on cpu 0:
 mem_cgroup_select_victim_node+0xb5/0x3d0 mm/memcontrol.c:1686
 try_to_free_mem_cgroup_pages+0x175/0x4c0 mm/vmscan.c:3376
 reclaim_high.constprop.0+0xf7/0x140 mm/memcontrol.c:2349
 mem_cgroup_handle_over_high+0x96/0x180 mm/memcontrol.c:2430
 tracehook_notify_resume include/linux/tracehook.h:197 [inline]
 exit_to_usermode_loop+0x20c/0x2c0 arch/x86/entry/common.c:163
 prepare_exit_to_usermode+0x180/0x1a0 arch/x86/entry/common.c:194
 swapgs_restore_regs_and_return_to_usermode+0x0/0x40

read to 0xffff88809fade9b0 of 4 bytes by task 7290 on cpu 1:
 mem_cgroup_select_victim_node+0x92/0x3d0 mm/memcontrol.c:1675
 try_to_free_mem_cgroup_pages+0x175/0x4c0 mm/vmscan.c:3376
 reclaim_high.constprop.0+0xf7/0x140 mm/memcontrol.c:2349
 mem_cgroup_handle_over_high+0x96/0x180 mm/memcontrol.c:2430
 tracehook_notify_resume include/linux/tracehook.h:197 [inline]
 exit_to_usermode_loop+0x20c/0x2c0 arch/x86/entry/common.c:163
 prepare_exit_to_usermode+0x180/0x1a0 arch/x86/entry/common.c:194
 swapgs_restore_regs_and_return_to_usermode+0x0/0x40

mem_cgroup_select_victim_node() can be called concurrently which reads
and modifies memcg->last_scanned_node without any synchrnonization. So,
read and modify memcg->last_scanned_node with READ_ONCE()/WRITE_ONCE()
to stop potential reordering.

Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Suggested-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: Greg Thelen <gthelen@xxxxxxxxxx>
Reported-by: syzbot+13f93c99c06988391efe@xxxxxxxxxxxxxxxxxxxxxxxxx
---
 mm/memcontrol.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c4c555055a72..5a06739dd3e4 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1667,7 +1667,7 @@ int mem_cgroup_select_victim_node(struct mem_cgroup *memcg)
 	int node;
 
 	mem_cgroup_may_update_nodemask(memcg);
-	node = memcg->last_scanned_node;
+	node = READ_ONCE(memcg->last_scanned_node);
 
 	node = next_node_in(node, memcg->scan_nodes);
 	/*
@@ -1678,7 +1678,7 @@ int mem_cgroup_select_victim_node(struct mem_cgroup *memcg)
 	if (unlikely(node == MAX_NUMNODES))
 		node = numa_node_id();
 
-	memcg->last_scanned_node = node;
+	WRITE_ONCE(memcg->last_scanned_node, node);
 	return node;
 }
 #else
-- 
2.24.0.rc0.303.g954a862665-goog