linux-kernel - Re: [PATCH] swiotlb: Fix inversed DMA_ATTR_NO

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Date:   Fri, 25 May 2018 10:41:20 +0200
From:   Christoph Hellwig <hch@....de>
To:     christian.koenig@....com
Cc:     Michel Dänzer <michel@...nzer.net>,
        Christoph Hellwig <hch@....de>,
        iommu@...ts.linux-foundation.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH] swiotlb: Fix inversed DMA_ATTR_NO_WARN test

On Tue, May 22, 2018 at 03:13:58PM +0200, Christian König wrote:
> Am 02.05.2018 um 18:59 schrieb Michel Dänzer:
>> On 2018-05-02 06:21 PM, Christoph Hellwig wrote:
>>> On Wed, May 02, 2018 at 04:31:09PM +0200, Michel Dänzer wrote:
>>>>> No.  __GFP_NOWARN (and gfp_t flags in general) are the wrong interface
>>>>> for dma allocations and just cause problems.  I actually plan to
>>>>> get rid of the gfp_t argument in dma_alloc_attrs sooner, and only
>>>>> allow either GFP_KERNEL or GFP_DMA passed in dma_alloc_coherent.
>>>> How about GFP_TRANSHUGE_LIGHT? TTM uses that to opportunistically
>>>> allocate huge pages (GFP_TRANSHUGE can result in unacceptably long
>>>> delays with memory pressure).
>>> Well, that is exactly what I don't want drivers to do - same for
>>> __GFP_COMP in some drm code.  This very much assumes the page allocator
>>> is used to back dma allocations, which very often it actually isn't, and
>>> any use of magic gfp flags creates a tight coupling of consumers with a
>>> specific implementation.
>>>
>>> In general I can't think of a good reason not to actually use
>>> GFP_TRANSHUGE_LIGHT by default in the dma allocator unless
>>> DMA_ATTR_ALLOC_SINGLE_PAGES is set.  Can you prepare a patch for that?
>> I'm afraid I'll have to leave that to somebody else.
>
> Coming back to this topic once more, sorry for the delay but busy as usual 
> :)
>
> What exactly do you mean with "dma allocator" here? The TTM allocator using 
> the dma_alloc_coherent calls? Or the swiotlb implementation of the calls?

dma allocatr in this case: backends for dma_alloc_coherent/
dma_alloc_attrs.  Most importantly dma_direct_alloc.

But while we're at it I can't actually see any GFP_TRANSHUGE_LIGHT
usage in TTM, just plain old GFP_TRANSHUGE.