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


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