On Tue, Nov 03 2015, James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote:
> From: James Bottomley <JBottomley@xxxxxxxx>
>
> It was noticed that we lose precision in the final calculation for some
> inputs. The most egregious example is size=3000 blk_size=1900 in units of 10
> should yield 5.70 MB but in fact yields 3.00 MB (oops). This is because the
> current algorithm doesn't correctly account for all the remainders in the
> logarithms. Fix this by doing a correct calculation in the remainders based
> on napier's algorithm. Additionally, now we have the correct result, we have
> to account for arithmetic rounding because we're printing 3 digits of
> precision. This means that if the fourth digit is five or greater, we have to
> round up, so add a section to ensure correct rounding. Finally account for
> all possible inputs correctly, including zero for block size.
>
> Reported-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # delay backport by two months for testing
> Fixes: b9f28d863594c429e1df35a0474d2663ca28b307
> Signed-off-by: James Bottomley <JBottomley@xxxxxxxx>
>
> --
>
> v2: updated with a recommendation from Rasmus Villemoes to truncate the
> initial precision at just under 32 bits
>
> diff --git a/lib/string_helpers.c b/lib/string_helpers.c
> index 5939f63..363faca 100644
> --- a/lib/string_helpers.c
> +++ b/lib/string_helpers.c
> @@ -43,38 +43,40 @@ void string_get_size(u64 size, u64 blk_size, const enum string_size_units units,
> [STRING_UNITS_10] = 1000,
> [STRING_UNITS_2] = 1024,
> };
> - int i, j;
> - u32 remainder = 0, sf_cap, exp;
> + static const unsigned int rounding[] = { 500, 50, 5, 0};
j necessarily ends up being 0, 1 or 2. Any reason to include the last entry?
> +
> + while (blk_size >= UINT_MAX)
> i++;
> - }
>
> - exp = divisor[units] / (u32)blk_size;
> - /*
> - * size must be strictly greater than exp here to ensure that remainder
> - * is greater than divisor[units] coming out of the if below.
> - */
> - if (size > exp) {
> - remainder = do_div(size, divisor[units]);
> - remainder *= blk_size;
> + while (size >= UINT_MAX)
> i++;
Please spell it U32_MAX. Also, it's not clear why you left out the
do_divs ;-)
Rasmus
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