Messages in this thread | | | From | Tomasz Figa <> | Date | Tue, 27 Jun 2017 21:44:20 +0900 | Subject | Re: [PATCH 1/2] iommu/dma: Respect __GFP_DMA and __GFP_DMA32 in incoming GFP flags |
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On Tue, Jun 27, 2017 at 9:26 PM, Robin Murphy <robin.murphy@arm.com> wrote: > On 27/06/17 12:17, Tomasz Figa wrote: >> On Tue, Jun 27, 2017 at 8:01 PM, Robin Murphy <robin.murphy@arm.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?
Best regards, Tomasz
> > Robin. > >> >> Best regards, >> Tomasz >> >>> >>> Robin. >>> >>>> Signed-off-by: Tomasz Figa <tfiga@chromium.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; >>>> >>> >
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