linux-kernel - Re: [RFCv3][PATCH 1/4] replace string_get

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Date:	Tue, 04 Oct 2011 12:35:42 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Joe Perches <joe@...ches.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	rientjes@...gle.com, James.Bottomley@...senPartnership.com,
	hpa@...or.com
Subject: Re: [RFCv3][PATCH 1/4] replace string_get_size() arrays

On Sat, 2011-10-01 at 12:33 -0700, Joe Perches wrote:
> On Fri, 2011-09-30 at 17:08 -0700, Dave Hansen wrote:
> > Instead of explicitly storing the entire string for each
> > possible units, just store the thing that varies: the
> > first character.
> 
> trivia

I'm not sure what you mean by that.

> > diff -puN lib/string_helpers.c~string_get_size-pow2 lib/string_helpers.c
> > --- linux-2.6.git/lib/string_helpers.c~string_get_size-pow2	2011-09-30 16:50:31.628981352 -0700
> > +++ linux-2.6.git-dave/lib/string_helpers.c	2011-09-30 17:04:02.211607364 -0700
> > @@ -8,6 +8,23 @@
> >  #include <linux/module.h>
> >  #include <linux/string_helpers.h>
> >  
> > +static const char byte_units[] = "_KMGTPEZY";
> 
> u64 could be up to ~1.8**19 decimal
> zetta and yotta are not possible or necessary.
> u128 maybe someday, but then other changes
> would be necessary too.

Right, but we're only handling u64.

> > +static char *__units_str(enum string_size_units unit, char *buf, int index)
> > +{
> > +	int place = 0;
> > +
> > +	/* index=0 is plain 'B' with no other unit */
> > +	if (index) {
> > +		buf[place++] = byte_units[index];
> 
> index is unbounded (doesn't matter currently, it will for u128)

It's bound by the division or the log2 at least.  You do have to know
what you're passing in to __units_str, just like you had to know what
you were indexing with in to units_2[] and units_10[].

Is there something else you'd like to see done here?  We can
bounds-check index, but that seems a bit unnecessary since it's static
and the two callers are visible on the same page of code.

> > @@ -23,26 +40,19 @@
> >  int string_get_size(u64 size, const enum string_size_units units,
> >  		    char *buf, int len)
> []
> >  	const unsigned int divisor[] = {
> >  		[STRING_UNITS_10] = 1000,
> >  		[STRING_UNITS_2] = 1024,
> >  	};
> 
> static const or it might be better to use
> 	unsigned int divisor = (string_size_units == STRING_UNITS_2) ? 1024 : 1000;
> as that would make the code clearer in a
> couple of uses of divisor[] later.
> 
> > @@ -61,7 +71,7 @@ int string_get_size(u64 size, const enum
> >  	}
> >  
> >  	snprintf(buf, len, "%lld%s %s", (unsigned long long)size,
> 
> %llu

These two are about existing code, and not really necessary for this
set.  They'd make good follow-on patches, though.

-- Dave

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