Re: CMA: test_pages_isolated failures in alloc_contig_range

[Peter Hurley <peter@hurleysoftware.com>]

[ +cc Andrew Morton ]

On 10/28/2014 08:38 AM, Michal Nazarewicz wrote:
> On Sun, Oct 26 2014, Laurent Pinchart <laurent.pinchart@ideasonboard.com> 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.
> 
> Like Laura wrote, the message is not (should not be) a problem in
> itself:

[...]

> So as you can see cma_alloc will try another part of the cma region if
> test_pages_isolated fails.
> 
> Obviously, if CMA region is fragmented or there's enough space for only
> one allocation of required size isolation failures will cause allocation
> failures, so it's best to avoid them, but they are not always avoidable.
> 
> To debug you would probably want to add more debug information about the
> page (i.e. data from struct page) that failed isolation after the
> pr_warn in alloc_contig_range.

If the message does not indicate an actual problem, then its printk level is
too high. These messages have been reported when using 3.16+ distro kernels.

Regards,
Peter Hurley