linux-kernel - Re: [RFC] mmap(MAP

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Message-ID: <09210624-fb3a-4dc9-5720-49714dbef0f7@redhat.com>
Date:   Wed, 4 Oct 2017 14:29:08 -0700
From:   Laura Abbott <labbott@...hat.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Michal Nazarewicz <mina86@...a86.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org
Cc:     Marek Szyprowski <m.szyprowski@...sung.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Guy Shattah <sguy@...lanox.com>,
        Christoph Lameter <cl@...ux.com>
Subject: Re: [RFC] mmap(MAP_CONTIG)

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 déjà vu.  Wasn’t 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.

Thanks,
Laura