linux-kernel - Re: [11/14] vcompound: Fallbacks for order 1 stack allocations on IA64 and x86

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Date:	Tue, 25 Mar 2008 08:51:06 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Christoph Lameter <clameter@....com>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, viro@....linux.org.uk
Subject: Re: [11/14] vcompound: Fallbacks for order 1 stack allocations on IA64 and x86

On Mon, Mar 24, 2008 at 12:53:19PM -0700, Christoph Lameter wrote:
> On Fri, 21 Mar 2008, Andi Kleen wrote:
> 
> > The traditional reason this was discouraged (people seem to reinvent
> > variants of this patch all the time) was that there used 
> > to be drivers that did __pa() (or equivalent) on stack addresses
> > and that doesn't work with vmalloc pages.
> > 
> > I don't know if such drivers still exist, but such a change
> > is certainly not a no-brainer
> 
> I thought that had been cleaned up because some arches already have 

Someone posted a patch recently that showed that the cdrom layer
does it. Might be more. It is hard to audit a few million lines
of driver code.

> virtually mapped stacks? This could be debugged by testing with
> CONFIG_VFALLBACK_ALWAYS set. Which results in a stack that is always 
> vmalloc'ed and thus the driver should fail.

It might be a subtle failure.

Maybe sparse could be taught to check for this if it happens
in a single function? (cc'ing Al who might have some thoughts
on this). Of course if it happens spread out over multiple
functions sparse wouldn't help neither. 

-Andi
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