[PATCH v6 15/15] memory-hotplug: Do not allocate pdgat if it was not freed when offline.

[Tang Chen <tangchen@cn.fujitsu.com>]

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, we cannot
reset node_data and free pgdat when offlining a node. Just reset pgdat
to 0 and reuse the memory when the node is online again.

The problem is suggested by Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
The idea is from Wen Congyang <wency@cn.fujitsu.com>

NOTE: If we don't reset pgdat to 0, the WARN_ON in free_area_init_node()
      will be triggered.

Signed-off-by: Tang Chen <tangchen@cn.fujitsu.com>
Reviewed-by: Wen Congyang <wency@cn.fujitsu.com>
---
 mm/memory_hotplug.c |   20 ++++++++++++--------
 1 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 8b67752..8aa2b56 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1015,11 +1015,14 @@ static pg_data_t __ref *hotadd_new_pgdat(int nid, u64 start)
 	unsigned long zholes_size[MAX_NR_ZONES] = {0};
 	unsigned long start_pfn = start >> PAGE_SHIFT;
 
-	pgdat = arch_alloc_nodedata(nid);
-	if (!pgdat)
-		return NULL;
+	pgdat = NODE_DATA(nid);
+	if (!pgdat) {
+		pgdat = arch_alloc_nodedata(nid);
+		if (!pgdat)
+			return NULL;
 
-	arch_refresh_nodedata(nid, pgdat);
+		arch_refresh_nodedata(nid, pgdat);
+	}
 
 	/* we can use NODE_DATA(nid) from here */
 
@@ -1072,7 +1075,7 @@ out:
 int __ref add_memory(int nid, u64 start, u64 size)
 {
 	pg_data_t *pgdat = NULL;
-	int new_pgdat = 0;
+	int new_pgdat = 0, new_node = 0;
 	struct resource *res;
 	int ret;
 
@@ -1083,12 +1086,13 @@ int __ref add_memory(int nid, u64 start, u64 size)
 	if (!res)
 		goto out;
 
-	if (!node_online(nid)) {
+	new_pgdat = NODE_DATA(nid) ? 0 : 1;
+	new_node = node_online(nid) ? 0 : 1;
+	if (new_node) {
 		pgdat = hotadd_new_pgdat(nid, start);
 		ret = -ENOMEM;
 		if (!pgdat)
 			goto error;
-		new_pgdat = 1;
 	}
 
 	/* call arch's memory hotadd */
@@ -1100,7 +1104,7 @@ int __ref add_memory(int nid, u64 start, u64 size)
 	/* we online node here. we can't roll back from here. */
 	node_set_online(nid);
 
-	if (new_pgdat) {
+	if (new_node) {
 		ret = register_one_node(nid);
 		/*
 		 * If sysfs file of new node can't create, cpu on the node
-- 
1.7.1