mm: fix handling of pagesets for downed cpus

After downing/upping a cpu, an attempt to set
/proc/sys/vm/percpu_pagelist_fraction results in an oops in
percpu_pagelist_fraction_sysctl_handler().

If a processor is downed then we need to set the pageset pointer back to
the boot pageset.

Updates of the high water marks should not access pagesets of unpopulated
zones (those pointer go to the boot pagesets which would be no longer
functional if their size would be increased beyond zero).

Signed-off-by: Dimitri Sivanich <[email protected]>
Signed-off-by: Christoph Lameter <[email protected]>
Reviewed-by: KOSAKI Motohiro <[email protected]>
Cc: Nick Piggin <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/page_alloc.c

@@ -3026,7 +3026,7 @@ static int __cpuinit process_zones(int cpu)
 		if (dzone == zone)
 			break;
 		kfree(zone_pcp(dzone, cpu));
-		zone_pcp(dzone, cpu) = NULL;
+		zone_pcp(dzone, cpu) = &boot_pageset[cpu];
 	}
 	return -ENOMEM;
 }
@@ -3041,7 +3041,7 @@ static inline void free_zone_pagesets(int cpu)
 		/* Free per_cpu_pageset if it is slab allocated */
 		if (pset != &boot_pageset[cpu])
 			kfree(pset);
-		zone_pcp(zone, cpu) = NULL;
+		zone_pcp(zone, cpu) = &boot_pageset[cpu];
 	}
 }
 
@@ -4659,7 +4659,7 @@ int percpu_pagelist_fraction_sysctl_handler(ctl_table *table, int write,
 	ret = proc_dointvec_minmax(table, write, file, buffer, length, ppos);
 	if (!write || (ret == -EINVAL))
 		return ret;
-	for_each_zone(zone) {
+	for_each_populated_zone(zone) {
 		for_each_online_cpu(cpu) {
 			unsigned long  high;
 			high = zone->present_pages / percpu_pagelist_fraction;