Re: [RFC PATCH 1/2] dma-buf/udmabuf: Introduce CONFIG_UDMABUF_SIZE_LIMIT_MBYTES

[Christian König]

On 7/11/26 16:48, Robert Mader wrote:
> As udmabuf increasingly enjoys popularity - being used in projects like
> libcamera, Gstreamer, Mesa and KWin - users more frequently encounter
> cases where the current default size limit of 64MB is too low. Examples
> include allocating video buffers at a 8K resolution - and even 4K is
> affected when using non-subsampled video formats or high bit depths.
> 
> While the limit can already be changed via the kernel command line,
> exposing it as a kernel config makes that easier and more discoverable
> for distros. Thus let's do that.

Well config options are usually only useful if the value can't be changed on runtime through a module parameter, but that is clearly not the case here.

On the other hand I do see your problem. I would just vote to disable the limit by default, there is nothing preventing userspace from allocating multiple uDMA-bufs so it doesn't seem to prevent any security issue or similar.

Regards,
Christian.


> 
> Signed-off-by: Robert Mader <robert.mader@xxxxxxxxxxxxx>
> ---
>  drivers/dma-buf/Kconfig   | 6 ++++++
>  drivers/dma-buf/udmabuf.c | 4 ++++
>  2 files changed, 10 insertions(+)
> 
> diff --git a/drivers/dma-buf/Kconfig b/drivers/dma-buf/Kconfig
> index 7efc0f0d0712..35f0779cdc80 100644
> --- a/drivers/dma-buf/Kconfig
> +++ b/drivers/dma-buf/Kconfig
> @@ -40,6 +40,12 @@ config UDMABUF
>           A driver to let userspace turn memfd regions into dma-bufs.
>           Qemu can use this to create host dmabufs for guest framebuffers.
> 
> +config UDMABUF_SIZE_LIMIT_MBYTES
> +       int "Size limit in Mega Bytes"
> +       default 64
> +       help
> +         Maximum size of a udmabuf, in megabytes. Default is 64.
> +
>  config DMABUF_DEBUG
>         bool "DMA-BUF debug checks"
>         depends on DMA_SHARED_BUFFER
> diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
> index bced421c0d65..a83153326362 100644
> --- a/drivers/dma-buf/udmabuf.c
> +++ b/drivers/dma-buf/udmabuf.c
> @@ -20,7 +20,11 @@ static int list_limit = 1024;
>  module_param(list_limit, int, 0644);
>  MODULE_PARM_DESC(list_limit, "udmabuf_create_list->count limit. Default is 1024.");
> 
> +#ifdef CONFIG_UDMABUF_SIZE_LIMIT_MBYTES
> +static int size_limit_mb = CONFIG_UDMABUF_SIZE_LIMIT_MBYTES;
> +#else
>  static int size_limit_mb = 64;
> +#endif
>  module_param(size_limit_mb, int, 0644);
>  MODULE_PARM_DESC(size_limit_mb, "Max size of a dmabuf, in megabytes. Default is 64.");
> 
> --
> 2.55.0
>