On Tue, 2015-11-03 at 23:13 +0100, Rasmus Villemoes wrote:
> On Tue, Nov 03 2015, James Bottomley <james.bottom...@hansenpartnership.com>
> wrote:
>
> > From: James Bottomley <jbottom...@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 <vkuzn...@redhat.com>
> > Cc: sta...@vger.kernel.org # delay backport by two months for testing
> > Fixes: b9f28d863594c429e1df35a0474d2663ca28b307
> > Signed-off-by: James Bottomley <jbottom...@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?
No reason beyond a vague worry someone might try to increase the printed
precision by one digit.
> > +
> > + 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
Why? there's no reason not to use the arithmetic UINT_MAX here. Either
works, of course but UINT_MAX is standard.
> . Also, it's not clear why you left out the
> do_divs ;-)
Over reduction.
James
> Rasmus
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