Re: [PATCH] virtio_mem: add bitmap_test_range_all_set/zero helpers

["David Hildenbrand (Arm)" <david@kernel.org>]

On 4/5/26 23:32, Luan Haickel Araujo wrote:
> Encapsulate find_next_bit and find_next_zero_bit calls into helpers
> to verify if a range of subblocks is entirely plugged or unplugged.
> 
> This improves readability as requested by the TODO comments in the
> source code.
> 
> Validation confirms the change is binary-neutral on ARM64, with the
> generated instructions remaining identical.
> 
> Suggested-by: David Hildenbrand <david@redhat.com>
> 
> Signed-off-by: Luan Haickel Araujo <luanharaujo@gmail.com>
> ---
> Hi David,
> 

Hi!

> This is my first patch contribution to the Linux kernel, following up on
> your TODO notes in virtio_mem.c.
> 
> As part of my learning process (LFX 103), I verified the change in a
> QEMU (KVM) environment with test_bitmap. I also compared the generated
> assembly on ARM64 and it remains identical, confirming the helpers are
> binary-neutral as expected.
> 
> I noticed these helpers could also be useful in include/linux/bitmap.h in
> the future. If you think it makes sense to move them there, I'd be happy
> to work on that in a next step.

Right, the TODO really indicated that core code should provide such
helpers, not virtio-mem itself. There is no real benefit in having these
local helpers.

> 
> Looking forward to your feedback.
> 
> Luan.
>  drivers/virtio/virtio_mem.c | 24 ++++++++++++++++++------
>  1 file changed, 18 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_mem.c b/drivers/virtio/virtio_mem.c
> index 48051e9e98ab..e94e30af3a5d 100644
> --- a/drivers/virtio/virtio_mem.c
> +++ b/drivers/virtio/virtio_mem.c
> @@ -538,6 +538,22 @@ static void virtio_mem_sbm_set_sb_unplugged(struct virtio_mem *vm,
>  	__bitmap_clear(vm->sbm.sb_states, bit, count);
>  }
>  
> +static inline bool bitmap_test_range_all_set(const unsigned long *map,
> +					     unsigned int start, unsigned int nbits)
> +{
> +	unsigned int next_zero_bit = find_next_zero_bit(map, start + nbits, start);
> +
> +	return next_zero_bit >= start + nbits;
> +}
> +
> +static inline bool bitmap_test_range_all_zero(const unsigned long *map,
> +					      unsigned int start, unsigned int nbits)
> +{
> +	unsigned int next_set_bit = find_next_bit(map, start + nbits, start);
> +
> +	return next_set_bit >= start + nbits;
> +}

If you look closely, fs/btrfs/misc.h provides these functions. So they
should probably be factored out into common code.

Then, virtio-mem could just naturally reuse them.

But, I wonder whether we want to call them

	bitmap_test_set()
	bitmap_test_clear()

Given that they have the exact same interface as
bitmap_set/bitmap_clear, consuming a range.

-- 
Cheers,

David