linux-kernel - Re: [RFC Patch] use MTRR for write combining if PAT is not available

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Date:	Thu, 22 Oct 2009 21:14:13 +0900
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Thomas Schlichter <thomas.schlichter@....de>
CC:	Suresh Siddha <suresh.b.siddha@...el.com>,
	Jan Beulich <JBeulich@...ell.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	"dri-devel@...ts.sourceforge.net" <dri-devel@...ts.sourceforge.net>,
	Robert Hancock <hancockrwd@...il.com>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	"jbarnes@...tuousgeek.org" <jbarnes@...tuousgeek.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Thomas Hellstrom <thellstrom@...are.com>,
	Tejun Heo <tj@...nel.org>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	"x86@...nel.org" <x86@...nel.org>, Yinghai Lu <yinghai@...nel.org>
Subject: Re: [RFC Patch] use MTRR for write combining if PAT is not available

On 10/22/2009 09:08 PM, Thomas Schlichter wrote:
>>
>> I have couple of issues with this patchset still. pci_mmap_page_range()
>> doesn't get called for each fork(). So, we won't be ref counting the
>> mtrr usage properly.
>
> When forking, what happens with the "struct file"? If it is being copied, then the
> processes share the same private data which would be freed during the first
> release(). I think this would be a problem whereever file-private data are used.
>
> So I think it must be shared between the forked processes and some reference
> counting must exist. This reference counting must ensure that release() is only
> called when all processes did close() their file.
>
> And in that case (shared "struct file", one single release() call in the end) this
> implementation should be completely safe...
>

struct file is shared between forked processes.

	-hpa
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