Re: [PATCH v1 4/4] bitmap: Make bitmap_fill() and bitmap_zero() consistent

Yury Norov
2018-01-10T08:49:57Z

On Tue, Jan 09, 2018 at 07:24:30PM +0200, Andy Shevchenko wrote:
> Behaviour of bitmap_fill() differs from bitmap_zero() in a way
> how bits behind bitmap are handed. bitmap_zero() clears entire bitmap
> by unsigned long boundary, while bitmap_fill() mimics bitmap_set().
>
> Here we change bitmap_fill() behaviour to be consistent with bitmap_zero()
> and add a note to documentation.
>
> The change might reveal some bugs in the code where unused bits handled
> differently and in such cases bitmap_set() has to be used.

There is only 51 users of bitmap_fill() in the kernel, including
tests. If you propose this change, I think you'd check them all
manually. Sorry that.

Also, there's tools/include/linux/bitmap.h which has a copy of
bitmap_fill(), and should be consistent with main kernel sources.

> Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> ---
> include/linux/bitmap.h | 15 ++++++++++-----
> 1 file changed, 10 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> index a17b5121d4f5..49aea0b37994 100644
> --- a/include/linux/bitmap.h
> +++ b/include/linux/bitmap.h
> @@ -67,6 +67,11 @@
> * bitmap_from_arr32(dst, buf, nbits) Copy nbits from u32[] buf to dst
> * bitmap_to_arr32(buf, src, nbits) Copy nbits from buf to u32[] dst
> *
> + * Note, bitmap_zero() and bitmap_fill() operate over the region of
> + * unsigned longs, that is, bits behind bitmap till the unsigned long
> + * boundary will be zeroed or filled as well. Consider to use
> + * bitmap_clear() or bitmap_set() to make explicit zeroing or filling
> + * respectively.
> */
>
> /**
> @@ -206,12 +211,12 @@ static inline void bitmap_zero(unsigned long *dst, unsigned int nbits)
>
> static inline void bitmap_fill(unsigned long *dst, unsigned int nbits)
> {
> - unsigned int nlongs = BITS_TO_LONGS(nbits);
> - if (!small_const_nbits(nbits)) {
> - unsigned int len = (nlongs - 1) * sizeof(unsigned long);
> - memset(dst, 0xff, len);
> + if (small_const_nbits(nbits))
> + *dst = ~0UL;
> + else {
> + unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
> + memset(dst, 0xff, len);
> }
> - dst[nlongs - 1] = BITMAP_LAST_WORD_MASK(nbits);
> }
>
> static inline void bitmap_copy(unsigned long *dst, const unsigned long *src,
> --
> 2.15.1