Subject: [PATCH] iommu/arm-smmu: fix leak in arm_smmu_flush_pgtable

From: [email-masked] (Mitchel Humpherys)
Date: Thu, 05 Mar 2015 09:28:02 -0800

On Thu, Mar 05 2015 at 02:38:45 AM, Robin Murphy <[email-masked]> wrote:
> Hi Mitch,
>
> On 05/03/15 00:18, Mitchel Humpherys wrote:
>> We're currently mapping a page in arm_smmu_flush_pgtable without ever
>> unmapping it.  Fix this by calling dma_unmap_page on the returned dma
>> address.  Since the only reason we're calling dma_map_page is to make
>> sure it actually gets flushed out to RAM, we can just call
>> dma_unmap_page immediately following the map.
>>
>> Without this, eventually swiotlb runs out of memory and starts printing
>> things like:
>>
>>      [   35.545076] arm-smmu d00000.arm,smmu: swiotlb buffer is full (sz: 128 bytes)
>>
>
> So, you have non-coherent SMMUs too ;) The real problem is that the SMMU's
> DMA mask is wrong (as it happens I've just given Will a patch to fix that)
> - this is really just doing a whole bunch of unnecessary work (two memory
> copies and two cache flushes, one of which isn't even flushing the right
> area) to hide the problem. With an appropriate DMA mask set,
> swiotlb_map_page becomes a no-op and we fall through to the cache flush
> without ever allocating anything.

Yeah I noticed that as well...  But isn't this still incorrect usage of
the API (DMA-API-HOWTO.txt seems to indicate that calls to map should
always be balanced with calls to unmap)?  What we really want to do here
is just call __dma_map_area directly, but the comment on that guy
expressly forbids it...  Not sure what's worse, abusing the DMA API or
disobeying that comment?

-Mitch

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