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

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Date:   Tue, 27 Jun 2017 20:17:55 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Robin Murphy <robin.murphy@....com>
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 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). 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?

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;
>>
>