From: Arnd Bergmann <arnd@arndb.de>
To: linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 08/10] mm: cma: Contiguous Memory Allocator added
Date: Wed, 15 Jun 2011 09:37:18 +0200
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Cc: Marek Szyprowski <m.szyprowski@samsung.com>,
        "'Michal Nazarewicz'" <mina86@mina86.com>,
        "'Ankita Garg'" <ankita@in.ibm.com>,
        "'Daniel Walker'" <dwalker@codeaurora.org>,
        "'Jesse Barker'" <jesse.barker@linaro.org>,
        "'Mel Gorman'" <mel@csn.ul.ie>, linux-kernel@vger.kernel.org,
        linaro-mm-sig@lists.linaro.org, linux-mm@kvack.org,
        "'Kyungmin Park'" <kyungmin.park@samsung.com>,
        "'KAMEZAWA Hiroyuki'" <kamezawa.hiroyu@jp.fujitsu.com>,
        "'Andrew Morton'" <akpm@linux-foundation.org>,
        linux-media@vger.kernel.org
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On Wednesday 15 June 2011 09:11:39 Marek Szyprowski wrote:
> I see your concerns, but I really wonder how to determine the properties
> of the global/default cma pool. You definitely don't want to give all
> available memory o CMA, because it will have negative impact on kernel
> operation (kernel really needs to allocate unmovable pages from time to
> time). 

Exactly. This is a hard problem, so I would prefer to see a solution for
coming up with reasonable defaults.

> The only solution I see now is to provide Kconfig entry to determine
> the size of the global CMA pool, but this still have some issues,
> especially for multi-board kernels (each board probably will have
> different amount of RAM and different memory-consuming devices
> available). It looks that each board startup code still might need to
> tweak the size of CMA pool. I can add a kernel command line option for
> it, but such solution also will not solve all the cases (afair there
> was a discussion about kernel command line parameters for memory 
> configuration and the conclusion was that it should be avoided).

The command line option can be a last resort if the heuristics fail,
but it's not much better than a fixed Kconfig setting.

How about a Kconfig option that defines the percentage of memory
to set aside for contiguous allocations?

	Arnd
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