Re: CMA: test_pages_isolated failures in alloc_contig_range

[Laura Abbott]

On 10/26/2014 2:09 PM, Laurent Pinchart wrote:
> Hello,
>
> I've run into a CMA-related issue while testing a DMA engine driver with
> dmatest on a Renesas R-Car ARM platform.
>
> When allocating contiguous memory through CMA the kernel prints the following
> messages to the kernel log.
>
> [   99.770000] alloc_contig_range test_pages_isolated(6b843, 6b844) failed
> [  124.220000] alloc_contig_range test_pages_isolated(6b843, 6b844) failed
> [  127.550000] alloc_contig_range test_pages_isolated(6b845, 6b846) failed
> [  132.850000] alloc_contig_range test_pages_isolated(6b845, 6b846) failed
> [  151.390000] alloc_contig_range test_pages_isolated(6b843, 6b844) failed
> [  166.490000] alloc_contig_range test_pages_isolated(6b843, 6b844) failed
> [  181.450000] alloc_contig_range test_pages_isolated(6b845, 6b846) failed
>
> I've stripped the dmatest module down as much as possible to remove any
> hardware dependencies and came up with the following implementation.
...
> Loading the module will start 4 threads that will allocate and free DMA
> coherent memory in a tight loop and eventually produce the error. It seems
> like the probability of occurrence grows with the number of threads, which
> could indicate a race condition.
>
> The tests have been run on 3.18-rc1, but previous tests on 3.16 did exhibit
> the same behaviour.
>
> I'm not that familiar with the CMA internals, help would be appreciated to
> debug the problem.

Are you actually seeing allocation failures or is it just the messages?
The messages themselves may be harmless if the allocation is succeeding.
It's an indication that the particular range could not be isolated and
therefore another range should be used for the CMA allocation. Joonsoo
Kim had a patch series[1] that was designed to correct some problems with
isolation and from my testing it helps fix some CMA related errors. You
might try picking that up to see if it helps.

Thanks,
Laura

[1] https://lkml.org/lkml/2014/10/23/90

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