linux-kernel - Re: [PATCH/RFCv4 0/6] The Contiguous Memory Allocator framework

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Date:	Sat, 28 Aug 2010 16:16:09 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Hans Verkuil <hverkuil@...all.nl>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Nazarewicz <m.nazarewicz@...sung.com>,
	linux-mm@...ck.org, Daniel Walker <dwalker@...eaurora.org>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	Jonathan Corbet <corbet@....net>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Pawel Osciak <p.osciak@...sung.com>,
	Russell King <linux@....linux.org.uk>,
	Zach Pfeffer <zpfeffer@...eaurora.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-media@...r.kernel.org, Mel Gorman <mel@....ul.ie>
Subject: Re: [PATCH/RFCv4 0/6] The Contiguous Memory Allocator framework

On Sat, 2010-08-28 at 15:58 +0200, Hans Verkuil wrote:
> > Isn't the proposed CMA thing vulnerable to the exact same problem? If
> > you allow sharing of regions and plug some allocator in there you get
> > the same problem. If you can solve it there, you can solve it for any
> > kind of reservation scheme.
> 
> Since with cma you can assign a region exclusively to a driver you can ensure
> that this problem does not occur. Of course, if you allow sharing then you will
> end up with the same type of problem unless you know that there is only one
> driver at a time that will use that memory.

I think you could do the same thing, the proposed page allocator
solutions still needs to manage pageblock state, you can manage those
the same as you would your cma regions -- the difference is that you get
the option of letting the rest of the system use the memory in a
transparent manner if you don't need it.


> There is obviously a trade-off. I was just wondering how costly it is.
> E.g. would it be a noticeable delay making 64 MB memory available in this
> way on a, say, 600 MHz ARM. 

Right, dunno really, rather depends on the memory bandwidth of your arm
device I suspect. It is something you'd have to test. 

In case the machine isn't fast enough, there really isn't anything you
can do but keep the memory empty at all times; unless of course the
device in question needs it.
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