[PATCH] Fix up per-cpu page batch sizes

The code to clamp batch sizes to 2^n - 1 went missing and an extra
check got added, which must have been a hunk of the "higer order pcp
batch refills" work sneaking in.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 8 insertions, 8 deletions

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b257720edfc8..3b21a13d841c 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1772,16 +1772,16 @@ static int __devinit zone_batchsize(struct zone *zone)
                 batch = 1;
 
         /*
-         * We will be trying to allcoate bigger chunks of contiguous
-         * memory of the order of fls(batch).  This should result in
-         * better cache coloring.
+         * Clamp the batch to a 2^n - 1 value. Having a power
+         * of 2 value was found to be more likely to have
+         * suboptimal cache aliasing properties in some cases.
          *
-         * A sanity check also to ensure that batch is still in limits.
+         * For example if 2 tasks are alternately allocating
+         * batches of pages, one task can end up with a lot
+         * of pages of one half of the possible page colors
+         * and the other with pages of the other colors.
          */
-        batch = (1 << fls(batch + batch/2));
-
-        if (fls(batch) >= (PAGE_SHIFT + MAX_ORDER - 2))
-                batch = PAGE_SHIFT + ((MAX_ORDER - 1 - PAGE_SHIFT)/2);
+        batch = (1 << (fls(batch + batch/2)-1)) - 1;
 
         return batch;
 }