Re: mm: Initialise per_cpu_nodestats for all online pgdats at boot

On Thu, Aug 04, 2016 at 10:24:04AM +0100, Mel Gorman wrote:

[    1.713998] Unable to handle kernel paging request for data at address 0xff7a10000
[    1.714164] Faulting instruction address: 0xc000000000270cd0
[    1.714304] Oops: Kernel access of bad area, sig: 11 [#1]
[    1.714414] SMP NR_CPUS=2048 NUMA PowerNV
[    1.714530] Modules linked in:
[    1.714647] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.7.0-kvm+ #118
[    1.714786] task: c000000ff0680010 task.stack: c000000ff0704000
[    1.714926] NIP: c000000000270cd0 LR: c000000000270ce8 CTR: 0000000000000000
[    1.715093] REGS: c000000ff0707900 TRAP: 0300   Not tainted  (4.7.0-kvm+)
[    1.715232] MSR: 9000000102009033 <SF,HV,VEC,EE,ME,IR,DR,RI,LE,TM[E]>  CR: 846b6824  XER: 20000000
[    1.715748] CFAR: c000000000008768 DAR: 0000000ff7a10000 DSISR: 42000000 SOFTE: 1
GPR00: c000000000270d08 c000000ff0707b80 c0000000011fb200 0000000000000000
GPR04: 0000000000000800 0000000000000000 0000000000000000 0000000000000000
GPR08: ffffffffffffffff 0000000000000000 0000000ff7a10000 c00000000122aae0
GPR12: c000000000a1e440 c00000000fb80000 c00000000000c188 0000000000000000
GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
GPR20: 0000000000000000 0000000000000000 0000000000000000 c000000000cecad0
GPR24: c000000000d035b8 c000000000d6cd18 c000000000d6cd18 c000001fffa86300
GPR28: 0000000000000000 c000001fffa96300 c000000001230034 c00000000122eb18
[    1.717484] NIP [c000000000270cd0] refresh_zone_stat_thresholds+0x80/0x240
[    1.717568] LR [c000000000270ce8] refresh_zone_stat_thresholds+0x98/0x240
[    1.717648] Call Trace:
[    1.717687] [c000000ff0707b80] [c000000000270d08] refresh_zone_stat_thresholds+0xb8/0x240 (unreliable)

I've been investigating node hotplug. That path is also going to require 
initialization of per_cpu_nodestats. This worked for me:

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 3894b65..41266dc 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c

@@ -1219,6 +1219,7 @@ static pg_data_t __ref *hotadd_new_pgdat(int nid, u64 start)
  
  	/* init node's zones as empty zones, we don't have any present pages.*/
  	free_area_init_node(nid, zones_size, start_pfn, zholes_size);
+	pgdat->per_cpu_nodestats = alloc_percpu(struct per_cpu_nodestat);
  
  	/*
  	 * The node we allocated has no zone fallback lists. For avoiding

@@ -1249,6 +1250,7 @@ static pg_data_t __ref *hotadd_new_pgdat(int nid, u64 start)
  static void rollback_node_hotadd(int nid, pg_data_t *pgdat)
  {
  	arch_refresh_nodedata(nid, NULL);
+	free_percpu(pgdat->per_cpu_nodestats);
  	arch_free_nodedata(pgdat);
  	return;
  }

-- 
Reza Arbab