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

[Kasireddy, Vivek]

> Subject: Re: [PATCH v2] dma-buf/udmabuf: Disable the size limit by
> default
> 
> 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@xxxxxxxxxxxxx>
> >>
> >> ---
> >>
> >> 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@xxxxxxxxxxxxx/
> >> 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@xxxxxxx/
> >> 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@xxxxxxxxx>
> >
> > 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@xxxxxxx/
Yeah, looks like this change seems to be desirable in many use-cases.
I have pushed it to drm-misc-next today.

Thanks,
Vivek

> 
> >
> > Thanks,
> > Vivek
> >>   struct udmabuf {
> >>   	pgoff_t pagecount;
> >> --
> >> 2.55.0
> 
> --
> Robert Mader
> Consultant Software Developer
> 
> Collabora Ltd.
> Platinum Building, St John's Innovation Park, Cambridge CB4 0DS, UK
> Registered in England & Wales, no. 5513718