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

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Date:	Mon, 29 Sep 2014 08:09:47 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Akinobu Mita <akinobu.mita@...il.com>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Don Dutile <ddutile@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	x86@...nel.org, iommu@...ts.linux-foundation.org
Subject: Re: [PATCH v3 0/5] enhance DMA CMA on x86

On 09/27/2014 08:31 PM, Akinobu Mita wrote:
> 2014-09-27 23:30 GMT+09:00 Peter Hurley <peter@...leysoftware.com>:
>> 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
of DMA_ATTR_FORCE_CONTIGUOUS?


>> 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()?

Regards,
Peter Hurley


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