linux-kernel - Re: [PATCH] linux/bitmap.h: fix BITMAP_LAST_WORD

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Message-ID: <20180726093728.GA9069@yury-thinkpad>
Date:   Thu, 26 Jul 2018 12:37:28 +0300
From:   Yury Norov <ynorov@...iumnetworks.com>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        corbet@....net, linux@...musvillemoes.dk, dgilbert@...hat.com,
        mawilcox@...rosoft.com
Subject: Re: [PATCH] linux/bitmap.h: fix BITMAP_LAST_WORD_MASK

On Thu, Jul 26, 2018 at 04:07:51PM +0800, Wei Wang wrote:
> The existing BITMAP_LAST_WORD_MASK macro returns 0xffffffff if nbits is
> 0. This patch changes the macro to return 0 when there is no bit needs to
> be masked.

I think this is intentional behavour. Previous version did return ~0UL
explicitly in this case. See patch 89c1e79eb3023 (linux/bitmap.h: improve
BITMAP_{LAST,FIRST}_WORD_MASK) from Rasmus.

Introducing conditional branch would affect performance. All existing
code checks nbits for 0 before handling last word where needed
explicitly. So I think we'd better change nothing here.

Yury

> Signed-off-by: Wei Wang <wei.w.wang@...el.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Rasmus Villemoes <linux@...musvillemoes.dk>
> Cc: Yury Norov <ynorov@...iumnetworks.com>
> ---
>  include/linux/bitmap.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> index 1ee46f4..12af3d7 100644
> --- a/include/linux/bitmap.h
> +++ b/include/linux/bitmap.h
> @@ -194,7 +194,10 @@ extern int bitmap_print_to_pagebuf(bool list, char *buf,
>                                    const unsigned long *maskp, int nmaskbits);
> 
>  #define BITMAP_FIRST_WORD_MASK(start) (~0UL << ((start) & (BITS_PER_LONG - 1)))
> -#define BITMAP_LAST_WORD_MASK(nbits) (~0UL >> (-(nbits) & (BITS_PER_LONG - 1)))
> +#define BITMAP_LAST_WORD_MASK(nbits)                           \
> +(                                                              \
> +       nbits ? (~0UL >> (-(nbits) & (BITS_PER_LONG - 1))) : 0  \
> +)
> 
>  #define small_const_nbits(nbits) \
>         (__builtin_constant_p(nbits) && (nbits) <= BITS_PER_LONG)
> --
> 2.7.4