Re: [PATCH] memory hotplug: fix invalid memory access caused by stale kswapd pointer

[Minchan Kim <minchan@kernel.org>]

Hi,

On 06/14/2012 12:44 PM, Jiang Liu 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.
> 
> Signed-off-by: Xishi Qiu <qiuxishi@huawei.com>
> Signed-off-by: Jiang Liu <liuj97@gmail.com>


Reviewed-by: Minchan Kim <minchan@kernel.org>

Nitpick:

I saw kswapd_run and doubt why following line is there.

	if (pgdat->kswapd)
		return 0;

As looking thorough hotplug, I realized one can hotplug pages which are within different zones but same node.
Because kswapd live in per-node, that code is for checking kswapd already run. Right?

IMHO, better readable code is following as

diff --git a/include/linux/swap.h b/include/linux/swap.h
index b967eda..9425c0e 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -299,6 +299,7 @@ static inline void scan_unevictable_unregister_node(struct node *node)
 }
 #endif
 
+extern bool is_kswapd_running(int nid);
 extern int kswapd_run(int nid);
 extern void kswapd_stop(int nid);
 #ifdef CONFIG_CGROUP_MEM_RES_CTLR
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 0d7e3ec..60f9155 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -522,7 +522,8 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages)
        init_per_zone_wmark_min();
 
        if (onlined_pages) {
-               kswapd_run(zone_to_nid(zone));
+               if (!is_kswapd_running(zone_to_nid(zone))
+                       kswapd_run(zone_to_nid(zone));
                node_set_state(zone_to_nid(zone), N_HIGH_MEMORY);
        }
 
diff --git a/mm/vmscan.c b/mm/vmscan.c
index eeb3bc9..f331904 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2932,6 +2932,14 @@ static int __devinit cpu_callback(struct notifier_block *nfb,
        return NOTIFY_OK;
 }
 
+bool is_kswapd_running(int nid)
+{
+       pg_data_t *pgdat = NODE_DATA(nid);
+       if (pgdat->kswapd)
+               return true;
+       return false;
+}
+
 /*
  * This kswapd start function will be called by init and node-hot-add.
  * On node-hot-add, kswapd will moved to proper cpus if cpus are hot-added.
@@ -2941,9 +2949,6 @@ int kswapd_run(int nid)
        pg_data_t *pgdat = NODE_DATA(nid);
        int ret = 0;
 
-       if (pgdat->kswapd)
-               return 0;
-
        pgdat->kswapd = kthread_run(kswapd, pgdat, "kswapd%d", nid);
        if (IS_ERR(pgdat->kswapd)) {
                /* failure at boot is fatal */

Anyway, it's a preference and trivial but I hope you fix that, too if you don't mind
Of course, my nitpick shouldn't prevent merging your good fix.
If you mind it, I don't care of it. :)

Thanks.
-- 
Kind regards,
Minchan Kim

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