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Subject: Re: CMA region absolutely for a particular device?
To: Shawn Guo <shawnguo@kernel.org>, linux-kernel@vger.kernel.org,
        inux-arm-kernel@lists.infradead.org
References: <20160607125911.GA4279@tiger>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>, Arnd Bergmann <arnd@arndb.de>,
        Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>,
        Michal Nazarewicz <mina86@mina86.com>,
        Vlastimil Babka <vbabka@suse.cz>, Minchan Kim <minchan@kernel.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>
From: Laura Abbott <labbott@redhat.com>
Message-ID: <c4ddd222-9bab-e9d5-0c5c-2c02fd3813a8@redhat.com>
Date: Tue, 7 Jun 2016 08:55:54 -0700
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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