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

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@collabora.com

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