Messages in this thread | | | From | Rasmus Villemoes <> | Subject | Re: [PATCH v2] string_helpers: fix precision loss for some inputs | Date | Tue, 03 Nov 2015 23:13:46 +0100 |
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On Tue, Nov 03 2015, James Bottomley <James.Bottomley@HansenPartnership.com> wrote:
> From: James Bottomley <JBottomley@Odin.com> > > 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@redhat.com> > Cc: stable@vger.kernel.org # delay backport by two months for testing > Fixes: b9f28d863594c429e1df35a0474d2663ca28b307 > Signed-off-by: James Bottomley <JBottomley@Odin.com> > > -- > > 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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