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

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Date:   Thu, 28 Jan 2021 12:17:42 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Paul Gortmaker <paul.gortmaker@...driver.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Li Zefan <lizefan@...wei.com>, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Triplett <josh@...htriplett.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: Re: [PATCH 3/8] lib: bitmap: fold nbits into region struct

On Thu, Jan 28, 2021 at 2:52 AM Yury Norov <yury.norov@...il.com> wrote:
> On Wed, Jan 27, 2021 at 12:02 AM Paul Gortmaker
> <paul.gortmaker@...driver.com> wrote:

...

> > 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."
>
> struct region is purely internal structure. It's declared on stack and filled
> field-by-field using helpers. 'Someone' misusing the structure doesn't exist
> because the structure doesn't exist out of the scope.
>
> > 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.

> Region is a convenient structure. Adding nbits into it helps to remove
> validation
> logic from bitmap_set_region(), so it's worth doing this.
>
> Can you please have it unchanged?

I have a compromise proposal here, i.e. why not to create a wrapper
structure like

struct bitmap_region {
  unsigned int nbits;
  struct region r;
};

?

At least it will solve my concern and still be a local structure on
the stack without adding new parameters to called functions.

-- 
With Best Regards,
Andy Shevchenko