Re: node-hotplug: is memset 0 safe in try_offline_node()?
From: Kamezawa Hiroyuki
Date: Wed Mar 04 2015 - 04:04:12 EST
On 2015/03/04 17:03, Xishi Qiu wrote:
On 2015/3/4 11:56, Gu Zheng wrote:
Hi Xishi,
On 03/04/2015 10:52 AM, Xishi Qiu wrote:
On 2015/3/4 10:22, Xishi Qiu wrote:
On 2015/3/3 18:20, Gu Zheng wrote:
Hi Xishi,
On 03/03/2015 11:30 AM, Xishi Qiu wrote:
When hot-remove a numa node, we will clear pgdat,
but is memset 0 safe in try_offline_node()?
It is not safe here. In fact, this is a temporary solution here.
As you know, pgdat is accessed lock-less now, so protection
mechanism (RCUï) is needed to make it completely safe here,
but it seems a bit over-kill.
Hi Gu,
Can we just remove "memset(pgdat, 0, sizeof(*pgdat));" ?
I find this will be fine in the stress test except the warning
when hot-add memory.
As you see, it will trigger the warning in free_area_init_node().
Could you try the following patch? It will reset the pgdat before reuse it.
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 1778628..0717649 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1092,6 +1092,9 @@ static pg_data_t __ref *hotadd_new_pgdat(int nid, u64 start)
return NULL;
arch_refresh_nodedata(nid, pgdat);
+ } else {
+ /* Reset the pgdat to reuse */
+ memset(pgdat, 0, sizeof(*pgdat));
}
Hi Gu,
If schedule last a long time, next_zone may be still access the pgdat here,
so it is not safe enough, right?
How about just reseting pgdat->nr_zones and pgdat->classzone_idx to be 0 rather than
memset() ?
It seems breaking pointer information in pgdat is not a choice.
Just proper "values" should be reset.
Thanks,
-Kame
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