Messages in this thread Patch in this message | | | Date | Tue, 10 Jan 2012 22:54:55 +0800 | From | Wu Fengguang <> | Subject | [PATCH] lib: proportion: lower PROP_MAX_SHIFT to 32 on 64-bit kernel |
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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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