Re: CMA region absolutely for a particular device?

[Laura Abbott <labbott@redhat.com>]

On 06/07/2016 05:59 AM, Shawn Guo wrote:
> Hi,
>
> I'm using a separate CMA region than the system default one for
> a particular device to avoid fragmentation.  It does help.  But under
> certain circumstance (memory shortage), it seems some of the pages in
> the region will be used by system.  The really bad thing is that when
> a CMA allocation tries to move these occupied pages around, it just
> fails to do so with messages like "alloc_contig_range: ... PFNs busy".
> These pages thus become holes in a contiguous block and prevent the
> allocation from succeeding.
>
> Is it possible to make a CMA absolutely for a particular device, and
> even system movable pages cannot use the memory?  I can reserve a memory
> region from kernel and manage it with some custom and private interface
> for that particular device.  But obviously, the standard dma-mapping API
> and established underneath CMA infrastructural is more desirable to use,
> right?
>
> This is an arm64 device running on 4.1 kernel.  Any comments or
> suggestions will be appreciated.  Thanks.
>
> Shawn
>

You could try backporting the ZONE_CMA patches
http://article.gmane.org/gmane.linux.kernel.mm/152016 . The primary problem
there was CMA utilization but there was some discussion about migration
success as well http://article.gmane.org/gmane.linux.kernel/2228789

If that doesn't work for you, I think the 'reusable' property can be dropped
and it should just become a DMA coherent pool. I haven't tested this though.

Thanks,
Laura