Re: [RFC] mmap(MAP_CONTIG)

From: Laura Abbott
Date: Wed Oct 04 2017 - 17:29:17 EST

On 10/04/2017 10:08 AM, Mike Kravetz wrote:
> On 10/04/2017 04:54 AM, Michal Nazarewicz wrote:
>> On Tue, Oct 03 2017, Mike Kravetz wrote:
>>> At Plumbers this year, Guy Shattah and Christoph Lameter gave a presentation
>>> titled 'User space contiguous memory allocation for DMA' [1]. The slides
>>> point out the performance benefits of devices that can take advantage of
>>> larger physically contiguous areas.
>> Issue I have is that kind of memory needed may depend on a device. Some
>> may require contiguous blocks. Some may support scatter-gather. Some
>> may be behind IO-MMU and not care either way.
>> Furthermore, I feel dÃjà vu. Wasnât dmabuf supposed to address this
>> issue?
> Thanks Michal,
> I was unaware of dmabuf and am just now looking at capabilities. The
> question is whether or not the IB driver writers requesting mmap(MAP_CONTIG)
> functionality could make use of dmabuf. That is out of my are of expertise,
> so I will let them reply.

I don't think dmabuf as it exists today would help anything here.
It's designed to share buffers via fd but you still need some
place/driver to actually get the allocation and then export it
since there isn't a single interface for allocations. You could
convert drivers to take a dma_buf fd if there were appropriate
buffers available though.