Re: [PATCH] vfio: pci: use kzalloc_flex

[Alex Williamson]

[Cc +Leon]

On Wed, 25 Mar 2026 19:37:47 -0700
Rosen Penev <rosenp@xxxxxxxxx> wrote:

> Simplify allocation by using a flexible array member and kzalloc_flex.
> Less memory management needed.
> 
> Use __counted_by for extra runtime analysis. Move assignment to after
> allocation as required by __counted_by.

I don't understand this statement, nr_ranges was previously set after
the allocation of phys_vec.  The only reordering was relative to
setting vdev, but that appears arbitrary.

In fact, we don't need to explicitly set the __counted_by variable at
all, kzalloc_flex() handles that.  So if anything, it's now redundant.

Leon, any other comments?  This should have a v2 removing the
redundancy and fixing the commit log.

NB. This will be a bit messy to merge since kref and completion exist in
linux-next via drm, but maybe Linus will consolidate the hole in the
structure when he resolves it.  Thanks,

Alex

> 
> Signed-off-by: Rosen Penev <rosenp@xxxxxxxxx>
> ---
>  drivers/vfio/pci/vfio_pci_dmabuf.c | 18 +++++-------------
>  1 file changed, 5 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
> index 3a803923141b..40e7e035a720 100644
> --- a/drivers/vfio/pci/vfio_pci_dmabuf.c
> +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
> @@ -14,12 +14,12 @@ struct vfio_pci_dma_buf {
>  	struct vfio_pci_core_device *vdev;
>  	struct list_head dmabufs_elm;
>  	size_t size;
> -	struct phys_vec *phys_vec;
>  	struct p2pdma_provider *provider;
>  	u32 nr_ranges;
>  	struct kref kref;
>  	struct completion comp;
>  	u8 revoked : 1;
> +	struct phys_vec phys_vec[] __counted_by(nr_ranges);
>  };
>  
>  static int vfio_pci_dma_buf_attach(struct dma_buf *dmabuf,
> @@ -95,7 +95,6 @@ static void vfio_pci_dma_buf_release(struct dma_buf *dmabuf)
>  		up_write(&priv->vdev->memory_lock);
>  		vfio_device_put_registration(&priv->vdev->vdev);
>  	}
> -	kfree(priv->phys_vec);
>  	kfree(priv);
>  }
>  
> @@ -258,33 +257,28 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
>  	if (ret)
>  		goto err_free_ranges;
>  
> -	priv = kzalloc_obj(*priv);
> +	priv = kzalloc_flex(*priv, phys_vec, get_dma_buf.nr_ranges);
>  	if (!priv) {
>  		ret = -ENOMEM;
>  		goto err_free_ranges;
>  	}
> -	priv->phys_vec = kzalloc_objs(*priv->phys_vec, get_dma_buf.nr_ranges);
> -	if (!priv->phys_vec) {
> -		ret = -ENOMEM;
> -		goto err_free_priv;
> -	}
>  
> -	priv->vdev = vdev;
>  	priv->nr_ranges = get_dma_buf.nr_ranges;
> +	priv->vdev = vdev;
>  	priv->size = length;
>  	ret = vdev->pci_ops->get_dmabuf_phys(vdev, &priv->provider,
>  					     get_dma_buf.region_index,
>  					     priv->phys_vec, dma_ranges,
>  					     priv->nr_ranges);
>  	if (ret)
> -		goto err_free_phys;
> +		goto err_free_priv;
>  
>  	kfree(dma_ranges);
>  	dma_ranges = NULL;
>  
>  	if (!vfio_device_try_get_registration(&vdev->vdev)) {
>  		ret = -ENODEV;
> -		goto err_free_phys;
> +		goto err_free_priv;
>  	}
>  
>  	exp_info.ops = &vfio_pci_dmabuf_ops;
> @@ -323,8 +317,6 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
>  	dma_buf_put(priv->dmabuf);
>  err_dev_put:
>  	vfio_device_put_registration(&vdev->vdev);
> -err_free_phys:
> -	kfree(priv->phys_vec);
>  err_free_priv:
>  	kfree(priv);
>  err_free_ranges: