Re: [PATCH v2] dma-buf/udmabuf: Disable the size limit by default

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

On 28.07.26 22:20, Kasireddy, Vivek wrote:
> Hi Robert,
>
>> Subject: [PATCH v2] dma-buf/udmabuf: Disable the size limit by default
>>
>> As udmabuf increasingly enjoys popularity - being used in projects like
>> libcamera, Gstreamer, Mesa, KWin and Weston - 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 and high bit
>> depths.
>>
>> In its current form the size limit for individual buffers does not seem to
>> provide any additional level of protection - such as limiting the amount
>> of
>> memory a process can pin - as the later can just allocate multiple
>> buffers.
>> If additional guardrails are desired, they would likely require some kind
>> accounting not limited to individual buffers.
>>
>> Therefor let's disable the size limit by default by setting it to the
>> maximal possible value, INT_MAX.
>>
>> Signed-off-by: Robert Mader <robert.mader@collabora.com>
>>
>> ---
>>
>> Changes in V2:
>>   - Use INT_MAX instead of 0 in order to not change behavior otherwise.
>>
>> See
>> https://lore.kernel.org/dri-devel/20260711144814.8205-1-
>> robert.mader@collabora.com/
>> for a previous attempt to make the value configurable via kconfig - and
>> in particular
>> https://lore.kernel.org/dri-devel/6764ca6f-b4d8-4baa-9d27-
>> 2ca867ac2d41@amd.com/
>> for the suggestion and discussion to remove the default limit.
>> ---
>>   drivers/dma-buf/udmabuf.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
>> index bced421c0d65..639e93704924 100644
>> --- a/drivers/dma-buf/udmabuf.c
>> +++ b/drivers/dma-buf/udmabuf.c
>> @@ -20,9 +20,9 @@ static int list_limit = 1024;
>>   module_param(list_limit, int, 0644);
>>   MODULE_PARM_DESC(list_limit, "udmabuf_create_list->count limit.
>> Default is 1024.");
>>
>> -static int size_limit_mb = 64;
>> +static int size_limit_mb = INT_MAX;
>>   module_param(size_limit_mb, int, 0644);
>> -MODULE_PARM_DESC(size_limit_mb, "Max size of a dmabuf, in
>> megabytes. Default is 64.");
>> +MODULE_PARM_DESC(size_limit_mb, "Max size of a dmabuf, in
>> megabytes. Default is INT_MAX.");
> Acked-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
>
> If there are no further concerns/questions from anyone, I'll push it to
> drm-misc-next soon.

FTR., we already have a first duplicate :P

https://lore.kernel.org/dri-devel/20260803144120.11524-1-xaver.hugl@kde.org/

>
> Thanks,
> Vivek
>>   struct udmabuf {
>>   	pgoff_t pagecount;
>> --
>> 2.55.0

-- 
Robert Mader
Consultant Software Developer

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