Re: A regression in recent 3.2 kernel: bdi_dirty_limit() divide error

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

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On Sun, Jan 08, 2012 at 11:19:14AM +0100, Peter Zijlstra wrote:

> But even then, we limit PROP_MAX_SHIFT to 3*BITS_PER_LONG/4, I don't
> think that could ever overflow.

do_div() only uses the lower 32 bit value of the 64 bit denominator,
which may happen to be 0.

The denominator is not really 64 bit, but limited by PROP_MAX_SHIFT =
48 bit, however that upper limit looks not enough.

Ilya, would you help try the attached patch instead?

Thanks,
Fengguang

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Subject: 
Date: Mon Jan 09 11:53:50 CST 2012


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)