linux-kernel - Re: [PATCH v1 1/1] bitmap.h: add const modifier to bitmap_next_[set/clear]

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Message-ID: <CAN8YU5PD9i=qztMLyuFsMQFf80pz04dLOSQ0NJ9=uHb1bTh52w@mail.gmail.com>
Date:   Wed, 28 Jul 2021 08:39:15 +0200
From:   Andrea Merello <andrea.merello@...il.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andrea Merello <andrea.merello@....it>
Subject: Re: [PATCH v1 1/1] bitmap.h: add const modifier to
 bitmap_next_[set/clear]_region() argument

Il giorno mar 27 lug 2021 alle ore 17:26 Yury Norov
<yury.norov@...il.com> ha scritto:
>
> On Tue, Jul 27, 2021 at 02:48:42PM +0300, Andy Shevchenko wrote:
> > On Tue, Jul 27, 2021 at 11:44:41AM +0200, Andrea Merello wrote:
> > > Those two functions don't modify the bitmap, so their bitmap argument
> > > should be const. This patch add this.
>
> Thanks Andrea.
>
> Acked-by: Yury Norov <yury.norov@...il.com>
>
> > Constification is always a good thing. No objections from me,
> > although Yuri is doing something with them in one of his patch series.
>
> I try to remove those two because in practice they bring more mess
> than good. All real use-cases for bitmap_next_{set,clear}_region
> relate to iterating the whole bitmap, ie nobody just wants to find a
> next region. Untill recently there was only a single user of the API,
> so I easily reworked the code to use find_first_bit/find_next bit and
> by chance return faster.

Ah, that's interesting to know: I'm working on a new driver that used
find_next_zero_bit() and find_next_bit() [0]; Andy told me that my
algorithm could be rewritten exploiting some bitmap_* helpers, so I
rewrote my code using bitmap_for_each_set_region(), which in turn uses
bitmap_next_set_region().

Should I revert to my previous implementation that uses
find_next_zero_bit() and find_next_bit() ?

[0] https://lore.kernel.org/linux-iio/20210715141742.15072-3-andrea.merello@gmail.com/

> https://github.com/norov/linux/commit/1c870b5c3fcd2eea9b351a1e0af8d1e93be78e1e
>
> Recently in next-20210716, there appeared another user in fs/btrfs/extent_io.c:
> find_next_dirty_byte(). The fun is that in that case the length of bitmap
> is 16 bit, so it's probably simpler to return the bitmap by value,
> instead of calling prologue code up to 8 times. Anyways, I'll contact
> authors of the find_next_dirty_byte() and ask if it's possible to rework
> their code.
>
> > Yuri, do you have a public repo / branch that people can base their changes to
> > bitmap stuff against of?
>
> I collected not yet upstreamed bitmap patches here:
>
> https://github.com/norov/linux/commits/bitmap-20210716

Looks like also bitmap_for_each_set_region() went away indeed ?

> Please let me know if you believe we need a more official branch for
> bitmaps. I'd encourage people to review what we already have and
> upstream it.
>
> Thanks,
> Yury
>
> > Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> >
> > > Signed-off-by: Andrea Merello <andrea.merello@....it>
> > > ---
> > >  include/linux/bitmap.h | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> > > index a36cfcec4e77..ea4a8f1a2545 100644
> > > --- a/include/linux/bitmap.h
> > > +++ b/include/linux/bitmap.h
> > > @@ -458,7 +458,7 @@ static inline void bitmap_replace(unsigned long *dst,
> > >             __bitmap_replace(dst, old, new, mask, nbits);
> > >  }
> > >
> > > -static inline void bitmap_next_clear_region(unsigned long *bitmap,
> > > +static inline void bitmap_next_clear_region(const unsigned long *bitmap,
> > >                                         unsigned int *rs, unsigned int *re,
> > >                                         unsigned int end)
> > >  {
> > > @@ -466,7 +466,7 @@ static inline void bitmap_next_clear_region(unsigned long *bitmap,
> > >     *re = find_next_bit(bitmap, end, *rs + 1);
> > >  }
> > >
> > > -static inline void bitmap_next_set_region(unsigned long *bitmap,
> > > +static inline void bitmap_next_set_region(const unsigned long *bitmap,
> > >                                       unsigned int *rs, unsigned int *re,
> > >                                       unsigned int end)
> > >  {
> > > --
> > > 2.17.1
> > >
> >
> > --
> > With Best Regards,
> > Andy Shevchenko
> >