Hi Robert,
Subject: Re: [RFC PATCH 1/2] dma-buf/udmabuf: Introduce CONFIG_UDMABUF_SIZE_LIMIT_MBYTES
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.
AFAIU, the main motivation behind the limits was to ensure that there are some kind of guardrails. I am not opposed to their removal but I think it would now allow users to pin large arbitrary amounts of memory and I feel like there needs to be a way to prevent users from doing that.
Early RFC versions of udmabuf had mlock accounting to address this issue but not sure why it was dropped eventually. I am wondering if it makes sense to bring that back.
Thanks, Vivek
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 64helpMaximum 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
linaro-mm-sig@lists.linaro.org
-
Kasireddy, Vivek