[PATCH] lib: proportion: lower PROP_MAX_SHIFT to 32 on 64-bit kernel

[Wu Fengguang <wfg@linux.intel.com>]

PROP_MAX_SHIFT should be set to <=32 on 64-bit box. This fixes two bugs
in the below lines of bdi_dirty_limit():

	bdi_dirty *= numerator;
	do_div(bdi_dirty, denominator);

1) divide error: do_div() only uses the lower 32 bit of the denominator,
   which may trimmed to be 0 when PROP_MAX_SHIFT > 32.

2) overflow: (bdi_dirty * numerator) could easily overflow if numerator
   used up to 48 bits, leaving only 16 bits to bdi_dirty

Tested-by: Ilya Tumaykin <librarian_rus@yahoo.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 include/linux/proportions.h |    4 ++++
 1 file changed, 4 insertions(+)

--- linux.orig/include/linux/proportions.h	2012-01-09 11:51:19.000000000 +0800
+++ linux/include/linux/proportions.h	2012-01-09 11:53:47.000000000 +0800
@@ -81,7 +81,11 @@ void prop_inc_percpu(struct prop_descrip
  * Limit the time part in order to ensure there are some bits left for the
  * cycle counter and fraction multiply.
  */
+#if BITS_PER_LONG == 32
 #define PROP_MAX_SHIFT (3*BITS_PER_LONG/4)
+#else
+#define PROP_MAX_SHIFT (BITS_PER_LONG/2)
+#endif
 
 #define PROP_FRAC_SHIFT		(BITS_PER_LONG - PROP_MAX_SHIFT - 1)
 #define PROP_FRAC_BASE		(1UL << PROP_FRAC_SHIFT)