[PATCH] virtio_mem: add bitmap_test_range_all_set/zero helpers

[PATCH] virtio_mem: add bitmap_test_range_all_set/zero helpers

[Luan Haickel Araujo]

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,

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.

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;
+}
+
 /*
  * Test if all selected subblocks are plugged.
  */
@@ -550,9 +566,7 @@ static bool virtio_mem_sbm_test_sb_plugged(struct virtio_mem *vm,
 	if (count == 1)
 		return test_bit(bit, vm->sbm.sb_states);
 
-	/* TODO: Helper similar to bitmap_set() */
-	return find_next_zero_bit(vm->sbm.sb_states, bit + count, bit) >=
-	       bit + count;
+	return bitmap_test_range_all_set(vm->sbm.sb_states, bit, count);
 }
 
 /*
@@ -564,9 +578,7 @@ static bool virtio_mem_sbm_test_sb_unplugged(struct virtio_mem *vm,
 {
 	const int bit = virtio_mem_sbm_sb_state_bit_nr(vm, mb_id, sb_id);
 
-	/* TODO: Helper similar to bitmap_set() */
-	return find_next_bit(vm->sbm.sb_states, bit + count, bit) >=
-	       bit + count;
+	return bitmap_test_range_all_zero(vm->sbm.sb_states, bit, count);
 }
 
 /*
-- 
2.53.0

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

[David Hildenbrand (Arm)]

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