Re: [PATCH] btrfs-progs: utils: negative numbers are more plausible than sizes over 8 EiB

[Zygo Blaxell <ce3g8jdj@umail.furryterror.org>]

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On Sat, Dec 03, 2016 at 10:25:17AM -0800, Omar Sandoval wrote:
> On Sat, Dec 03, 2016 at 01:19:38AM -0500, Zygo Blaxell wrote:
> > I got tired of seeing "16.00EiB" whenever btrfs-progs encounters a
> > negative size value.
> > 
> > e.g. during filesystem shrink we see:
> > 
> > Unallocated:
> >    /dev/mapper/testvol0   16.00EiB
> > 
> > Interpreting this as a signed quantity is much more useful:
> > 
> > Unallocated:
> >    /dev/mapper/testvol0  -26.29GiB
> > 
> > Signed-off-by: Zygo Blaxell <ce3g8jdj@umail.furryterror.org>
> > ---
> >  utils.c | 13 ++++++++-----
> >  1 file changed, 8 insertions(+), 5 deletions(-)
> > 
> > diff --git a/utils.c b/utils.c
> > index 69b580a..bd2b66e 100644
> > --- a/utils.c
> > +++ b/utils.c
> > @@ -2594,20 +2594,23 @@ static const char* unit_suffix_binary[] =
> >  static const char* unit_suffix_decimal[] =
> >  	{ "B", "kB", "MB", "GB", "TB", "PB", "EB"};
> >  
> > -int pretty_size_snprintf(u64 size, char *str, size_t str_size, unsigned unit_mode)
> > +int pretty_size_snprintf(u64 usize, char *str, size_t str_size, unsigned unit_mode)
> >  {
> >  	int num_divs;
> >  	float fraction;
> > -	u64 base = 0;
> > +	s64 base = 0;
> >  	int mult = 0;
> >  	const char** suffix = NULL;
> > -	u64 last_size;
> > +	s64 last_size;
> >  
> >  	if (str_size == 0)
> >  		return 0;
> >  
> > +	/* Negative numbers are more plausible than sizes over 8 EiB. */
> > +	s64 size = (s64)usize;
> 
> Just make pretty_size_snprintf() take an s64 size so it's clear from the
> function signature that it's signed instead of hidden in the definition.

I intentionally buried the unsigned -> signed conversion in the lowest
level function so I wouldn't trigger signed/unsigned conversion warnings
at all 46 call sites for pretty_size_mode.  The btrfs code uses u64
endemically for all size data, and I wasn't about to try to change that.

The word "pretty" in the function name should imply that what comes out
is a possibly lossy transformation of what goes in.  Since "16.00EiB"
is much more lossy than "-29.96GiB", I believe I am merely reducing the
lossiness quantitatively rather than qualitatively.

On the other hand, the signed/unsigned warning isn't enabled by default
in this project.  I can certainly do it that way if you prefer.

> > +
> >  	if ((unit_mode & ~UNITS_MODE_MASK) == UNITS_RAW) {
> > -		snprintf(str, str_size, "%llu", size);
> > +		snprintf(str, str_size, "%lld", size);
> >  		return 0;
> >  	}
> >  
> > @@ -2642,7 +2645,7 @@ int pretty_size_snprintf(u64 size, char *str, size_t str_size, unsigned unit_mod
> >  			   num_divs = 0;
> >  			   break;
> >  	default:
> > -		while (size >= mult) {
> > +		while ((size < 0 ? -size : size) >= mult) {
> >  			last_size = size;
> >  			size /= mult;
> >  			num_divs++;
> > -- 
> > 2.1.4
> > 
> > --
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> 

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