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: <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Reported-by: Ilya Tumaykin <[email protected]>
Tested-by: Ilya Tumaykin <[email protected]>
Signed-off-by: Wu Fengguang <[email protected]>

0 files changed, 0 insertions, 0 deletions