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.
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>