[patch 06/31] mm/page_alloc.c: fix calculation of pgdat->nr_zones

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From: Wei Yang <richard.weiyang@xxxxxxxxx>
Subject: mm/page_alloc.c: fix calculation of pgdat->nr_zones

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).

Michal said:

: Having an incorrect nr_zones might result in all sorts of problems which
: would be quite hard to debug (e.g.  reclaim not considering the movable
: zone).  I do not expect many users would suffer from this it but still
: this is trivial and obviously right thing to do so backporting to the
: stable tree shouldn't be harmful (last famous words).

Link: http://lkml.kernel.org/r/20181117022022.9956-1-richard.weiyang@xxxxxxxxx
Fixes: f1dd2cd13c4b ("mm, memory_hotplug: do not associate hotadded memory to zones until online")
Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
Cc: Anshuman Khandual <anshuman.khandual@xxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 mm/page_alloc.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/mm/page_alloc.c~mm-page_alloc-fix-calculation-of-pgdat-nr_zones
+++ a/mm/page_alloc.c
@@ -5813,8 +5813,10 @@ void __meminit init_currently_empty_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;
 
_



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