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