Re: [PATCH v3 1/4] lib/string_helpers: change blk_size to u32 for string_get_size() interface

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

On Tue, Nov 03 2015, James Bottomley <jbottomley@odin.com> wrote:

>
> It was a suggestion when I explained what the missing sources of
> precision were, I don't think it's really a suggestion when it comes
> with an exemplary patch.

ex·em·pla·ry
adjective

    1.
    serving as a desirable model; representing the best of its kind.

Said exemplary patch produces "1.10 KiB" for size=2047,
blk_size=1. (This is caused by the introduction of rounding, and is
probably fixable.)

James, I do understand the algorithm you're trying to use. What I don't
understand is why you insist on using the approach of reducing size and
blk_size all the way before multiplying them. It seems much simpler to
just reduce them till they're below U32_MAX (not keeping track of any
remainders at that point), multiply them, and then proceed as usual,
This avoids having to deal with weird cross-multiplication terms, gives
more accurate results (yes, I tested that) and avoids the extra 64/32
division you introduce by decrementing i.

Rasmus

To be precise, the body I suggest is

	while (blk_size > U32_MAX) {
		do_div(blk_size, divisor[units]);
		i++;
	}
	while (size > U32_MAX) {
		do_div(size, divisor[units]);
		i++;
	}
	size *= blk_size;
	while (size > divisor[units]) {
		remainder = do_div(size, divisor[units]);
		i++;
	}

whether the last one should be > or >= is debatable; I think 1024 KiB is
better than 1.00 MiB.