Re: node-hotplug: is memset 0 safe in try_offline_node()?

[Gu Zheng]

Hi Xishi,
Could you please try the following one?
It postpones the reset of obsolete pgdat from try_offline_node() to
hotadd_new_pgdat(), and just resetting pgdat->nr_zones and
pgdat->classzone_idx to be 0 rather than the whole reset by memset()
as Kame suggested.

Regards,
Gu

---
 mm/memory_hotplug.c |   13 ++++---------
 1 files changed, 4 insertions(+), 9 deletions(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 1778628..c17eebf 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1092,6 +1092,10 @@ static pg_data_t __ref *hotadd_new_pgdat(int nid, u64 start)
 			return NULL;
 
 		arch_refresh_nodedata(nid, pgdat);
+	} else {
+		/* Reset the nr_zones and classzone_idx to 0 before reuse */
+		pgdat->nr_zones = 0;
+		pgdat->classzone_idx = 0;
 	}
 
 	/* we can use NODE_DATA(nid) from here */
@@ -2021,15 +2025,6 @@ void try_offline_node(int nid)
 
 	/* notify that the node is down */
 	call_node_notify(NODE_DOWN, (void *)(long)nid);
-
-	/*
-	 * Since there is no way to guarentee the address of pgdat/zone is not
-	 * on stack of any kernel threads or used by other kernel objects
-	 * without reference counting or other symchronizing method, do not
-	 * reset node_data and free pgdat here. Just reset it to 0 and reuse
-	 * the memory when the node is online again.
-	 */
-	memset(pgdat, 0, sizeof(*pgdat));
 }
 EXPORT_SYMBOL(try_offline_node);
 
-- 
1.7.7



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()?
> 
> process A:			offline node XX:
> for_each_populated_zone()
> find online node XX
> cond_resched()
> 				offline cpu and memory, then try_offline_node()
> 				node_set_offline(nid), and memset(pgdat, 0, sizeof(*pgdat))
> access node XX's pgdat
> NULL pointer access error
> 
> Thanks,
> Xishi Qiu
> 
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