Re: [PATCH 1/2] lib: bitmap: add find_last_bit_range() and _find_last_bit_range()

[Michał Nazarewicz]

On Tue, May 12 2026, Yi Sun wrote:
> In some scenarios, it's not desirable to keep searching through the
> beginning of the bitmap, but rather to search within a specific part.
> The newly added function can accomplish this quickly.
>
> Signed-off-by: Yi Sun <yi.sun@xxxxxxxxxx>

Acked-by: Michał Nazarewicz <mina86@xxxxxxxxxx>

> ---
>  include/linux/find.h | 35 +++++++++++++++++++++++++++++++++++
>  lib/find_bit.c       | 30 ++++++++++++++++++++++++++++++
>  2 files changed, 65 insertions(+)
>
> diff --git a/include/linux/find.h b/include/linux/find.h
> index 6c2be8ca615d..7126b0fffe0f 100644
> --- a/include/linux/find.h
> +++ b/include/linux/find.h
> @@ -33,6 +33,8 @@ unsigned long _find_first_and_and_bit(const unsigned long *addr1, const unsigned
>  				      const unsigned long *addr3, unsigned long size);
>  extern unsigned long _find_first_zero_bit(const unsigned long *addr, unsigned long size);
>  extern unsigned long _find_last_bit(const unsigned long *addr, unsigned long size);
> +extern unsigned long _find_last_bit_range(const unsigned long *addr, unsigned long size,
> +					unsigned long offset);
>  
>  #ifdef __BIG_ENDIAN
>  unsigned long _find_first_zero_bit_le(const unsigned long *addr, unsigned long size);
> @@ -413,6 +415,39 @@ unsigned long find_last_bit(const unsigned long *addr, unsigned long size)
>  }
>  #endif
>  
> +#ifndef find_last_bit_range
> +/**
> + * find_last_bit_range - find the last set bit in a memory region
> + * @addr: The address to base the search on
> + * @size: The bitmap size in bits
> + * @offset: The bit number to start searching at
> + *
> + * Compared to the find_last_bit(),
> + * find_last_bit_range() has an additional parameter @offset,
> + * so it can search within a specific range of the bitmap,
> + * just like the find_next_bit().
> + *
> + * Returns the bit number of the last set bit, or size.
> + */
> +static __always_inline
> +unsigned long find_last_bit_range(const unsigned long *addr, unsigned long size,
> +				unsigned long offset)

This should be called find_last_bit_off instead.  `_range` suffix, to me
at least, implies that the order of arguments is (addr, offset, size).
`_off` is also the suffix used by `bitmap_find_next_zero_area_off` name.

> +{
> +	if (small_const_nbits(size)) {
> +		unsigned long val;
> +
> +		if (unlikely(offset >= size))
> +			return size;
> +
> +		val = *addr & GENMASK(size - 1, offset);
> +
> +		return val ? __fls(val) : size;
> +	}
> +
> +	return _find_last_bit_range(addr, size, offset);
> +}
> +#endif
> +
>  /**
>   * find_next_and_bit_wrap - find the next set bit in both memory regions
>   * @addr1: The first address to base the search on
> diff --git a/lib/find_bit.c b/lib/find_bit.c
> index 5ac52dfce730..bedc85053cea 100644
> --- a/lib/find_bit.c
> +++ b/lib/find_bit.c
> @@ -237,6 +237,36 @@ unsigned long _find_last_bit(const unsigned long *addr, unsigned long size)
>  EXPORT_SYMBOL(_find_last_bit);
>  #endif
>  
> +#ifndef find_last_bit_range
> +unsigned long _find_last_bit_range(const unsigned long *addr, unsigned long size,
> +				unsigned long offset)
> +{
> +	unsigned long val, idx, start_idx;
> +
> +	if (unlikely(offset >= size))
> +		return size;
> +
> +	val = BITMAP_LAST_WORD_MASK(size);
> +	idx = (size - 1) / BITS_PER_LONG;
> +	start_idx = offset / BITS_PER_LONG;
> +
> +	do {
> +		val &= addr[idx];
> +
> +		if (idx == start_idx)
> +			val &= BITMAP_FIRST_WORD_MASK(offset);
> +
> +		if (val)
> +			return idx * BITS_PER_LONG + __fls(val);
> +
> +		val = ~0UL;
> +	} while (idx-- > start_idx);

Perhaps:

```
	val = BITMAP_LAST_WORD_MASK(size) & addr[idx];
	while (!val && idx > start_idx)
		val = addr[--idx];
	if (idx == start_idx)
		val &= BITMAP_FIRST_WORD_MASK(offset);
	return val ? idx * BITS_PER_LONG + __fls(val) : size;
```

This moves the `idx == start_idx` condition outside of the loop so that
it’s not checked on each iteration.  It also removes the need for `val =
~0UL` from the loop again simplifying its body.

> +
> +	return size;
> +}
> +EXPORT_SYMBOL(_find_last_bit_range);
> +#endif
> +
>  unsigned long find_next_clump8(unsigned long *clump, const unsigned long *addr,
>  			       unsigned long size, unsigned long offset)
>  {

-- 
Best regards
ミハウ “𝓶𝓲𝓷𝓪86” ナザレヴィツ
«If at first you don’t succeed, give up skydiving»