Re: [PATCH v3 0/5] enhance DMA CMA on x86

From: Akinobu Mita
Date: Mon Sep 29 2014 - 10:33:01 EST

2014-09-29 21:09 GMT+09:00 Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>:
> On 09/27/2014 08:31 PM, Akinobu Mita wrote:
>> 2014-09-27 23:30 GMT+09:00 Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>:
>>> On 04/15/2014 09:08 AM, Akinobu Mita wrote:
>>>> This patch set enhances the DMA Contiguous Memory Allocator on x86.
> [...]
>>> What this patchset does is restrict all iommu configurations which can
>>> map all of system memory to one _very_ small physical region, thus disabling
>>> the whole point of an iommu.
>>> Now I know why my GPU is causing paging to disk! And why my RAID controller
>>> stalls for ages when I do a git log at the same time as a kernel build!
>> The solution I have for this is that instead of trying to
>> dma_alloc_from_contiguous() firstly, call alloc_pages() in dma_alloc_coherent().
>> dma_alloc_from_contiguous() should be called only when alloc_pages() is failed
>> or DMA_ATTR_FORCE_CONTIGUOUS is specified in dma_attr.
> Why is all this extra complexity being added when there are no X86 users

I misunderstood DMA_ATTR_FORCE_CONTIGUOUS. It is specified to request
that underlaying DMA mapping span physically contiguous with IOMMU.
But current alloc_dma_coherent() for intel-iommu always returns
physically contiguous memory, so it is ignored on x86.

>>> And the apparent goal of this patchset is to enable DMA allocation below
>>> 4GB, which is already supported in the existing page allocator with the
>>> GFP_DMA32 flag?!
>> The goal of this patchset is to enable huge DMA allocation which
>> alloc_pages() can't (> MAX_ORDER) for the devices that require it.
> What x86 devices need > MAX_ORDER DMA allocation and why can't they allocate
> directly from dma_alloc_from_contiguous()?

I need this for UFS unified memory extension which is apparently not in
mainline for now.

But there must be some other use cases on x86, too. Because I have
received several emails privately from developers who care its status.

And allocating directly from dma_alloc_from_contiguous() in the driver
doesn't work with IOMMU, as it just returns memory regoin and doesn't
create DMA mapping.
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