linux-kernel - Re: [PATCH 1/2] iommu/dma: Respect __GFP_DMA and __GFP

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Date:   Tue, 27 Jun 2017 15:52:55 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Tomasz Figa <tfiga@...omium.org>
Cc:     "open list:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH 1/2] iommu/dma: Respect __GFP_DMA and __GFP_DMA32 in
 incoming GFP flags

On 27/06/17 13:44, Tomasz Figa wrote:
> On Tue, Jun 27, 2017 at 9:26 PM, Robin Murphy <robin.murphy@....com> wrote:
>> On 27/06/17 12:17, Tomasz Figa wrote:
>>> On Tue, Jun 27, 2017 at 8:01 PM, Robin Murphy <robin.murphy@....com> wrote:
>>>> On 27/06/17 08:28, Tomasz Figa wrote:
>>>>> Current implementation of __iommu_dma_alloc_pages() keeps adding
>>>>> __GFP_HIGHMEM to GFP flags regardless of whether other zone flags are
>>>>> already included in the incoming flags. If __GFP_DMA or __GFP_DMA32 is
>>>>> set at the same time as __GFP_HIGHMEM, the allocation fails due to
>>>>> invalid zone flag combination.
>>>>>
>>>>> Fix this by checking for __GFP_DMA and __GFP_DMA32 in incoming GFP flags
>>>>> and adding __GFP_HIGHMEM only if they are not present.
>>>>
>>>> Wouldn't it make more sense to strip off the ZONE_DMA* related flags,
>>>> since the whole point here is that we don't care where the pages come from?
>>>
>>> I guess for my use case it wouldn't break things, as I strip them in
>>> my DMA mapping implementation right now (+/- one minor detail below).
>>>
>>> However I recall existing IOMMUs that could only use pages from within
>>> the 32-bit address space (e.g. Tegra X1).
>>
>> In general, iommu-dma can't really support IOMMUs which can't reach the
>> entirety of kernel memory - there's no easy way to determine what such a
>> limit is if it exists, nor necessarily enforce it, and either way the
>> streaming API callbacks are pretty much dead in the water.
> 
> Right. Especially with various user pointer sources it doesn't sound
> very realistic to enforce that all memory comes from __GFP_DMA(32)
> zone.
> 
> On the other hand, support for user pointer is optional in subsystems
> such as V4L2 and drivers for such disabled hardware could opt out.
> Then at least dma_alloc can be made working, giving some level of
> usability, IMHO higher than no IOMMU and contiguous memory allocated
> from CMA.
> 
>>
>>> Also the IOMMU I'm working
>>> on is a part of a PCI device and it might actually prefer 32-bit
>>> addressable memory as well (to avoid DAC addressing; I still need to
>>> evaluate this). With this said, maybe it could actually make sense to
>>> leave the choice to the DMA mapping implementation?
>>
>> I think you're right - we're just not in a position to make any decision
>> at this level, so we probably do have to do this for robustness. I would
>> like to fix the longstanding dodgy comment, though, to clarify that
>> "IOMMU can map any pages" is only an assumption, and particularly one
>> which is invalidated by the presence of GFP_DMA flags.
> 
> Just to make sure, should I resent with the commit updated? If so,
> what would be your preference on the wording?

Yes please; how about this?

	/*
	 * Unless we have some addressing limitation implied by GFP_DMA flags,
	 * assume the IOMMU can map all of RAM and we can allocate anywhere.
	 */
	if (!(gfp & (__GFP_DMA | __GFP_DMA32)))
		...

Feel free to reword it if you like, but those are the general lines I
was thinking along.

Robin.

> 
> Best regards,
> Tomasz
> 
>>
>> Robin.
>>
>>>
>>> Best regards,
>>> Tomasz
>>>
>>>>
>>>> Robin.
>>>>
>>>>> Signed-off-by: Tomasz Figa <tfiga@...omium.org>
>>>>> ---
>>>>>  drivers/iommu/dma-iommu.c | 10 ++++++++--
>>>>>  1 file changed, 8 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
>>>>> index 9d1cebe7f6cb..29965a092a69 100644
>>>>> --- a/drivers/iommu/dma-iommu.c
>>>>> +++ b/drivers/iommu/dma-iommu.c
>>>>> @@ -445,8 +445,14 @@ static struct page **__iommu_dma_alloc_pages(unsigned int count,
>>>>>       if (!pages)
>>>>>               return NULL;
>>>>>
>>>>> -     /* IOMMU can map any pages, so himem can also be used here */
>>>>> -     gfp |= __GFP_NOWARN | __GFP_HIGHMEM;
>>>>> +     /*
>>>>> +      * IOMMU can map any pages, so himem can also be used here,
>>>>> +      * unless another DMA zone is explicitly requested.
>>>>> +      */
>>>>> +     if (!(gfp & (__GFP_DMA | __GFP_DMA32)))
>>>>> +             gfp |= __GFP_HIGHMEM;
>>>>> +
>>>>> +     gfp |= __GFP_NOWARN;
>>>>>
>>>>>       while (count) {
>>>>>               struct page *page = NULL;
>>>>>
>>>>
>>