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

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

Ilya, would you post your /proc/vmstat?

I wonder if your proportion shift value is really large enough (> 32)
to trigger bug in this way.

Thanks,
Fengguang

On Tue, Jan 10, 2012 at 10:54:55PM +0800, Wu Fengguang wrote:
> 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)