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

[Michal Hocko <mhocko@kernel.org>]

On Mon 28-10-19 17:54:05, Shakeel Butt wrote:
> 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.

I am sorry but I do not understand the problem and the fix. Why does the
race happen and why does _ONCE fixes it? There is still no
synchronization. Do you want to prevent from memcg->last_scanned_node
reloading?

> Signed-off-by: Shakeel Butt <shakeelb@google.com>
> Suggested-by: Eric Dumazet <edumazet@google.com>
> Cc: Greg Thelen <gthelen@google.com>
> Reported-by: syzbot+13f93c99c06988391efe@syzkaller.appspotmail.com
> ---
>  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

-- 
Michal Hocko
SUSE Labs