mm/page_alloc: protect pcp->batch accesses with ACCESS_ONCE

pcp->batch could change at any point, avoid relying on it being a stable
value.

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>

1 files changed, 7 insertions, 4 deletions

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index eaaef2a09424..97b8f861e63d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1182,10 +1182,12 @@ void drain_zone_pages(struct zone *zone, struct per_cpu_pages *pcp)
 {
         unsigned long flags;
         int to_drain;
+        unsigned long batch;
 
         local_irq_save(flags);
-        if (pcp->count >= pcp->batch)
-                to_drain = pcp->batch;
+        batch = ACCESS_ONCE(pcp->batch);
+        if (pcp->count >= batch)
+                to_drain = batch;
         else
                 to_drain = pcp->count;
         if (to_drain > 0) {
@@ -1353,8 +1355,9 @@ void free_hot_cold_page(struct page *page, int cold)
                 list_add(&page->lru, &pcp->lists[migratetype]);
         pcp->count++;
         if (pcp->count >= pcp->high) {
-                free_pcppages_bulk(zone, pcp->batch, pcp);
-                pcp->count -= pcp->batch;
+                unsigned long batch = ACCESS_ONCE(pcp->batch);
+                free_pcppages_bulk(zone, batch, pcp);
+                pcp->count -= batch;
         }
 
 out: