Re: [Resend with ACK][PATCH] memory hotplug: fix invalid memoryaccess caused by stale kswapd pointer

[Andrew Morton]

On Wed, 20 Jun 2012 17:21:53 +0800
Jiang Liu <jiang.liu@xxxxxxxxxx> wrote:

> Function kswapd_stop() will be called to destroy the kswapd work thread
> when all memory of a NUMA node has been offlined. But kswapd_stop() only
> terminates the work thread without resetting NODE_DATA(nid)->kswapd to NULL.
> The stale pointer will prevent kswapd_run() from creating a new work thread
> when adding memory to the memory-less NUMA node again. Eventually the stale
> pointer may cause invalid memory access.

whoops.

>
> ...
>
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2961,8 +2961,10 @@ void kswapd_stop(int nid)
>  {
>  	struct task_struct *kswapd = NODE_DATA(nid)->kswapd;
>  
> -	if (kswapd)
> +	if (kswapd) {
>  		kthread_stop(kswapd);
> +		NODE_DATA(nid)->kswapd = NULL;
> +	}
>  }
>  
>  static int __init kswapd_init(void)

OK.

This function is full of races (ones which we'll never hit ;)) unless
the caller provides locking.  It appears that lock_memory_hotplug() is
the locking, so I propose this addition:

--- a/mm/vmscan.c~memory-hotplug-fix-invalid-memory-access-caused-by-stale-kswapd-pointer-fix
+++ a/mm/vmscan.c
@@ -2955,7 +2955,8 @@ int kswapd_run(int nid)
 }
 
 /*
- * Called by memory hotplug when all memory in a node is offlined.
+ * Called by memory hotplug when all memory in a node is offlined.  Caller must
+ * hold lock_memory_hotplug().
  */
 void kswapd_stop(int nid)
 {
--- a/include/linux/mmzone.h~memory-hotplug-fix-invalid-memory-access-caused-by-stale-kswapd-pointer-fix
+++ a/include/linux/mmzone.h
@@ -693,7 +693,7 @@ typedef struct pglist_data {
 					     range, including holes */
 	int node_id;
 	wait_queue_head_t kswapd_wait;
-	struct task_struct *kswapd;
+	struct task_struct *kswapd;	/* Protected by lock_memory_hotplug() */
 	int kswapd_max_order;
 	enum zone_type classzone_idx;
 } pg_data_t;
_


Also, I think kswapd_lock() and perhaps pglist_data.kswapd itself could
be placed under CONFIG_MEMORY_HOTPLUG to save a bit of space.

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