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

From: Andy Shevchenko <andy.shevchenko@gmail.com>
To: Yury Norov <yury.norov@gmail.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Li Zefan <lizefan@huawei.com>, Ingo Molnar <mingo@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Josh Triplett <josh@joshtriplett.org>,
	Peter Zijlstra <peterz@infradead.org>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>
Subject: Re: [PATCH 3/8] lib: bitmap: fold nbits into region struct
Date: Thu, 28 Jan 2021 12:17:42 +0200	[thread overview]
Message-ID: <CAHp75VcZvZrF=YGjQ7Jt2nHguo+kOLNuvrDUV9vHoLom4q+d6Q@mail.gmail.com> (raw)
In-Reply-To: <CAAH8bW-tWRJ4m3pP37YBVrferOa9CwMZGdZkSDC+GXvBfC=Uhw@mail.gmail.com>

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