linux-kernel - Re: [PATCH 3/8] lib: bitmap: fold nbits into region struct

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Date:   Wed, 27 Jan 2021 03:02:06 -0500
From:   Paul Gortmaker <paul.gortmaker@...driver.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, lizefan@...wei.com, mingo@...nel.org,
        tglx@...utronix.de, josh@...htriplett.org, yury.norov@...il.com,
        peterz@...radead.org, paulmck@...nel.org, fweisbec@...il.com,
        linux@...musvillemoes.dk
Subject: Re: [PATCH 3/8] lib: bitmap: fold nbits into region struct

[Re: [PATCH 3/8] lib: bitmap: fold nbits into region struct] On 26/01/2021 (Tue 23:16) Andy Shevchenko wrote:

> On Tue, Jan 26, 2021 at 12:11:36PM -0500, Paul Gortmaker wrote:
> > This will reduce parameter passing and enable using nbits as part
> > of future dynamic region parameter parsing.
> 
> One nit below, nevertheless
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> 
> > Cc: Yury Norov <yury.norov@...il.com>
> > Cc: Rasmus Villemoes <linux@...musvillemoes.dk>
> > Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> > Suggested-by: Yury Norov <yury.norov@...il.com>
> > Signed-off-by: Paul Gortmaker <paul.gortmaker@...driver.com>
> > ---
> >  lib/bitmap.c | 19 ++++++++++---------
> >  1 file changed, 10 insertions(+), 9 deletions(-)
> > 
> > diff --git a/lib/bitmap.c b/lib/bitmap.c
> > index 75006c4036e9..162e2850c622 100644
> > --- a/lib/bitmap.c
> > +++ b/lib/bitmap.c
> > @@ -487,24 +487,24 @@ EXPORT_SYMBOL(bitmap_print_to_pagebuf);
> >  
> >  /*
> >   * Region 9-38:4/10 describes the following bitmap structure:
> > - * 0	   9  12    18			38
> > - * .........****......****......****......
> > - *	    ^  ^     ^			 ^
> > - *      start  off   group_len	       end
> > + * 0	   9  12    18			38	     N
> > + * .........****......****......****..................
> > + *	    ^  ^     ^			 ^	     ^
> > + *      start  off   group_len	       end	 nbits
> >   */
> >  struct region {
> >  	unsigned int start;
> >  	unsigned int off;
> >  	unsigned int group_len;
> >  	unsigned int end;
> > +	unsigned int nbits;
> >  };
> >  
> > -static int bitmap_set_region(const struct region *r,
> > -				unsigned long *bitmap, int nbits)
> > +static int bitmap_set_region(const struct region *r, unsigned long *bitmap)
> >  {
> >  	unsigned int start;
> >  
> > -	if (r->end >= nbits)
> > +	if (r->end >= r->nbits)
> >  		return -ERANGE;
> >  
> >  	for (start = r->start; start <= r->end; start += r->group_len)
> > @@ -640,7 +640,8 @@ int bitmap_parselist(const char *buf, unsigned long *maskp, int nmaskbits)
> >  	struct region r;
> >  	long ret;
> >  
> > -	bitmap_zero(maskp, nmaskbits);
> > +	r.nbits = nmaskbits;
> 
> > +	bitmap_zero(maskp, r.nbits);
> 
> This sounds not right from style perspective.
> You have completely uninitialized r on stack, then you assign only one value
> for immediate use here and...

So, this change was added because Yury suggested that I "..store
nmaskbits in the struct region, and avoid passing nmaskbits as a
parameter."

To which I originally noted "I considered that and went with the param
so as to not open the door to someone possibly using an uninitialized
struct value later."

https://lore.kernel.org/lkml/20210122044357.GS16838@windriver.com/

Looking back, I had a similar thought as to yours, it seems...

I am also thinking more and more that nbits doesn't belong in the
region anyway - yes, a region gets validated against a specific nbits
eventually, but it doesn't need an nbits field to be a complete
specification.  The region "0-3" is a complete specification for "the
1st four cores" and is as valid on a 4 core machine as it is on a 64 core
machine -- a validation we do when we deploy the region on that machine.

I will set this change aside and get the nbits value to getnum() another
way, and leave the region struct as it was -- without a nbits field.

This will also resolve having the macro handling of region that you were
not really liking.

Paul.
--

> >  	while (buf) {
> >  		buf = bitmap_find_region(buf);
> > @@ -655,7 +656,7 @@ int bitmap_parselist(const char *buf, unsigned long *maskp, int nmaskbits)
> >  		if (ret)
> >  			return ret;
> >  
> > -		ret = bitmap_set_region(&r, maskp, nmaskbits);
> > +		ret = bitmap_set_region(&r, maskp);
> 
> ...hiding this fact here. Which I would expect that &r may be rewritten here.
> 
> I would leave these unchanged and simple assign the value in
> bitmap_set_region().
> 
> >  		if (ret)
> >  			return ret;
> >  	}
> > -- 
> > 2.17.1
> > 
> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 
>