From 3310225dfc71a35a2cc9340c15c0e08b14b3c754 Mon Sep 17 00:00:00 2001 From: Wu Fengguang Date: Mon, 9 Jan 2012 11:53:50 -0600 Subject: lib: proportion: lower PROP_MAX_SHIFT to 32 on 64-bit kernel 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 Cc: Cc: Peter Zijlstra Reported-by: Ilya Tumaykin Tested-by: Ilya Tumaykin Signed-off-by: Wu Fengguang --- include/linux/proportions.h | 4 ++++ 1 file changed, 4 insertions(+) (limited to 'include') diff --git a/include/linux/proportions.h b/include/linux/proportions.h index ef35bb73f69..26a8a4ed9b0 100644 --- a/include/linux/proportions.h +++ b/include/linux/proportions.h @@ -81,7 +81,11 @@ void prop_inc_percpu(struct prop_descriptor *pd, struct prop_local_percpu *pl) * 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) -- cgit v1.2.3-70-g09d2