linux-kernel - Re: Turn CONFIG_STRICT_DEVMEM in sysctl dev.mem.restricted

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Date:	Sun, 16 Nov 2008 08:06:41 -0800
From:	Arjan van de Ven <arjan@...radead.org>
To:	Bernhard Walle <bwalle@...e.de>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, crash-utility@...hat.com
Subject: Re: Turn CONFIG_STRICT_DEVMEM in sysctl dev.mem.restricted

On Sun, 16 Nov 2008 16:56:09 +0100
Bernhard Walle <bwalle@...e.de> wrote:

> > > While the original submission of CONFIG_STRICT_DEVMEM mentions
> > > that the option has been in RHEL and Fedora for 4 years without
> > > problems, that's only a half of the story. The truth is that at
> > > least RHEL has /dev/crash exactly to circumvent that /dev/mem
> > > restriction. Don't tell me that this is better than having that
> > > sysctl entry. ;-)
> > 
> > I assume /dev/crash is read only
> 
> I don't know. But if that matters, why can't we make /dev/mem
> write-only for certain areas and read-only for the rest ...?
> 
> > You either want this at compile time or you don't want it at all.
> 
> Why? You don't write something about my arguments (as Alan does, even
> though I disagree), you only write that you "nak" it.

It's very simple: if you want the strict form you really want the
strict form, not some half "oh but it's one line to turn off" form.


(Note: your usecase is in trouble in general already with PAT used by
modern video drivers: the requirement is that all mappings of the same
physical page are mapped with the same cachability semantics. mmaping
random parts of physical real memory via /dev/mem makes that a way too
complex issue and will likely turn the crash utility in what it's name
says ;-)

-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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