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

[Robert Mader <robert.mader@collabora.com>]

Hi,

On 13.07.26 11:12, Christian König wrote:
> 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.

I fully agree and would prefer that as well. If there is no resistance / 
concerns from anybody I'll send a corresponding patch at the end of the 
week.

Thanks,

Robert

>
> 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
>>