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

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Message-ID: <CAAH8bW-tWRJ4m3pP37YBVrferOa9CwMZGdZkSDC+GXvBfC=Uhw@mail.gmail.com>
Date:   Wed, 27 Jan 2021 16:47:50 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     Paul Gortmaker <paul.gortmaker@...driver.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        lizefan@...wei.com, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, josh@...htriplett.org,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>, fweisbec@...il.com,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: Re: [PATCH 3/8] lib: bitmap: fold nbits into region struct

On Wed, Jan 27, 2021 at 12:02 AM Paul Gortmaker
<paul.gortmaker@...driver.com> wrote:
>
> [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."

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.
>
> Paul.

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?

Thanks,
Yury