[PATCH] mm, page_alloc: fix calculation of pgdat->nr_zones

[Wei Yang <richard.weiyang@gmail.com>]

Function init_currently_empty_zone() will adjust pgdat->nr_zones and set
it to 'zone_idx(zone) + 1' unconditionally. This is correct in the
normal case, while not exact in hot-plug situation.

This function is used in two places:

  * free_area_init_core()
  * move_pfn_range_to_zone()

In the first case, we are sure zone index increase monotonically. While
in the second one, this is under users control.

One way to reproduce this is:
----------------------------

1. create a virtual machine with empty node1

   -m 4G,slots=32,maxmem=32G \
   -smp 4,maxcpus=8          \
   -numa node,nodeid=0,mem=4G,cpus=0-3 \
   -numa node,nodeid=1,mem=0G,cpus=4-7

2. hot-add cpu 3-7

   cpu-add [3-7]

2. hot-add memory to nod1

   object_add memory-backend-ram,id=ram0,size=1G
   device_add pc-dimm,id=dimm0,memdev=ram0,node=1

3. online memory with following order

   echo online_movable > memory47/state
   echo online > memory40/state

After this, node1 will have its nr_zones equals to (ZONE_NORMAL + 1)
instead of (ZONE_MOVABLE + 1).

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
---
 mm/page_alloc.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 5b7cd20dbaef..2d3c54201255 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5823,8 +5823,10 @@ void __meminit init_currently_empty_zone(struct zone *zone,
 					unsigned long size)
 {
 	struct pglist_data *pgdat = zone->zone_pgdat;
+	int zone_idx = zone_idx(zone) + 1;
 
-	pgdat->nr_zones = zone_idx(zone) + 1;
+	if (zone_idx > pgdat->nr_zones)
+		pgdat->nr_zones = zone_idx;
 
 	zone->zone_start_pfn = zone_start_pfn;
 
-- 
2.15.1