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    SubjectRe: CMA: test_pages_isolated failures in alloc_contig_range
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    Hi Laura,

    On Monday 27 October 2014 13:38:19 Laura Abbott wrote:
    > 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?

    It's just the messages, I haven't noticed allocation failures.

    > 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

    I've tested the patches but they don't seem to have any influence on the
    isolation test failures.

    --
    Regards,

    Laurent Pinchart


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