mm/page_alloc: prevent concurrent updaters of pcp ->batch and ->high

Because we are going to rely upon a careful transision between old and new
->high and ->batch values using memory barriers and will remove
stop_machine(), we need to prevent multiple updaters from interweaving
their memory writes.

Add a simple mutex to protect both update loops.

Signed-off-by: Cody P Schafer <cody@linux.vnet.ibm.com>
Cc: Gilad Ben-Yossef <gilad@benyossef.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d4bcc20ab6f00aa9b9aa598d5efa411734a6902f..8d4335779633f46d4d6a0ac067648029407f1961 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -65,6 +65,9 @@
 #include <asm/div64.h>
 #include "internal.h"
 
+/* prevent >1 _updater_ of zone percpu pageset ->high and ->batch fields */
+static DEFINE_MUTEX(pcp_batch_high_lock);
+
 #ifdef CONFIG_USE_PERCPU_NUMA_NODE_ID
 DEFINE_PER_CPU(int, numa_node);
 EXPORT_PER_CPU_SYMBOL(numa_node);
@@ -5557,6 +5560,8 @@ int percpu_pagelist_fraction_sysctl_handler(ctl_table *table, int write,
        ret = proc_dointvec_minmax(table, write, buffer, length, ppos);
        if (!write || (ret < 0))
                return ret;
+
+       mutex_lock(&pcp_batch_high_lock);
        for_each_populated_zone(zone) {
                for_each_possible_cpu(cpu) {
                        unsigned long  high;
@@ -5565,6 +5570,7 @@ int percpu_pagelist_fraction_sysctl_handler(ctl_table *table, int write,
                                per_cpu_ptr(zone->pageset, cpu), high);
                }
        }
+       mutex_unlock(&pcp_batch_high_lock);
        return 0;
 }
 
@@ -6078,7 +6084,9 @@ static int __meminit __zone_pcp_update(void *data)
 
 void __meminit zone_pcp_update(struct zone *zone)
 {
+       mutex_lock(&pcp_batch_high_lock);
        stop_machine(__zone_pcp_update, zone, NULL);
+       mutex_unlock(&pcp_batch_high_lock);
 }
 #endif