linux-kernel - Re: [PATCH] powerpc/mm: setting mmaped page cache property through device tree

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Date:	Tue, 1 Dec 2009 19:34:01 +0800
From:	Li Yang <leoli@...escale.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	paulus@...ba.org, linuxppc-dev@...abs.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/mm: setting mmaped page cache property through 
	device tree

On Tue, Dec 1, 2009 at 6:58 PM, Benjamin Herrenschmidt
<benh@...nel.crashing.org> wrote:
> On Tue, 2009-12-01 at 18:30 +0800, Li Yang wrote:
>> The patch adds the ability for powerpc architecture to set page cache
>> property of mmaped area through device tree.  This is useful for two
>> cases.  First, for memory shared with other OS'es to have the same cache
>> property to avoid cache paradoxes.  Second, enabling application to map
>> memory which is not managed by kernel as cacheable for better performance.
>
> But that doesn't solve the problem of those same pages being mapped
> cachable as part of the linear mapping does it ?

I think that it doesn't has this problem.  Only regions out of
lmb.memory are configurable through device tree.

>
> Can you tell us more about your precise usage scenario ? What are you

The scenario for the first case is that in a multicore system running
ASMP which means different OS runs on different cores.  They might
communicate through a shared memory region.  The region on every OS
need to be mapped with the same cache perperty to avoid cache paradox.

The scenario for the second case is to pre-allocate some memory to a
certain application or device (probably through mem=XXX kernel
parameter or limit through device tree).  The memory is not known to
kernel, but fully managed by the application/device.  We need being
able to map the region cachable for better performance.

> trying to achieve here ? We can find a solution though it might involve
> a specific driver to handle that memory.

Right, but what the user to kernel API should be used?  Is it ok to
use the O_SYNC flag as I previously proposed?

- Leo
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