linux-kernel - Re: d451564 breakage

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Date:	Sat, 14 Nov 2009 01:21:38 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Russell King <rmk+lkml@....linux.org.uk>
Cc:	Soeren Sandmann Pedersen <sandmann@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel List <linux-kernel@...r.kernel.org>
Subject: Re: d451564 breakage


* Russell King <rmk+lkml@....linux.org.uk> wrote:

> On Sat, Nov 14, 2009 at 12:17:14AM +0100, Ingo Molnar wrote:
> > 
> > * Russell King <rmk+lkml@....linux.org.uk> wrote:
> > 
> > > On Fri, Nov 13, 2009 at 11:48:20PM +0100, Ingo Molnar wrote:
> > > > 
> > > > * Russell King <rmk@....linux.org.uk> wrote:
> > > > 
> > > > > Change:
> > > > > 
> > > > > highmem: Fix debug_kmap_atomic() to also handle KM_IRQ_PTE, KM_NMI, and KM_NMI_PTE
> > > > > 
> > > > > Appears to break ARM:
> > > > > 
> > > > > mm/highmem.c: In function ???debug_kmap_atomic???:
> > > > > mm/highmem.c:436: error: ???KM_NMI??? undeclared (first use in this function)
> > > > 
> > > > indeed - sorry.
> > > > 
> > > > Note that debug_kmap_atomic() will be removed in v2.6.33 so i'd suggest 
> > > > to just do the easy solution and add #ifndef dummy definitions to 
> > > > mm/highmem.c to cover ARM - we'll remove the whole cruft for good.
> > > 
> > > Actually, the following patch is probably the simplest solution.
> > > 
> > > > Btw., testing sidenote: i test the ARM defconfig and it didnt break 
> > > > there. Perhaps highmem is off in the ARM defconfig? It would be helpful 
> > > > if the ARM defconfig enabled highmem.
> > > 
> > > Given that highmem on ARM is experimental, I'd rather not have it enabled
> > > in too many machine defconfigs as standard just yet.
> > > 
> > > However, enabling highmem on itself is not enough to show this breakage,
> > > you also need highmem debugging enabled.
> > > 
> > >  arch/arm/include/asm/kmap_types.h |    6 ++++++
> > >  1 files changed, 6 insertions(+), 0 deletions(-)
> > > 
> > > diff --git a/arch/arm/include/asm/kmap_types.h b/arch/arm/include/asm/kmap_types.h
> > > index d16ec97..c019949 100644
> > > --- a/arch/arm/include/asm/kmap_types.h
> > > +++ b/arch/arm/include/asm/kmap_types.h
> > > @@ -22,4 +22,10 @@ enum km_type {
> > >  	KM_TYPE_NR
> > >  };
> > >  
> > > +#ifdef CONFIG_DEBUG_HIGHMEM
> > > +#define KM_NMI		(-1)
> > > +#define KM_NMI_PTE	(-1)
> > > +#define KM_IRQ_PTE	(-1)
> > > +#endif
> > 
> > Please solve this in mm/highmem.c as Andrew suggested it - other 
> > architectures could be affected as well beyond ARM.
> 
> Have you actually put any thought into that approach? [...]

Yes.

> [...]  What you're suggesting means:
> 
> - adding preprocessor definitions to all existing places where these
>   symbols are defined
> - adding multiple complex ifdefs to mm/highmem.c thusly:
> 
> 		if (
> #ifdef KM_NMI
> 			type != KM_NMI
> #else
> 			1
> #endif
> 			&&
> #ifdef KM_NMI_PTE
> 			type != KM_NMI_PTE
> #else
> 			1
> #endif

I'd NAK such ugly code.

What i was thinking of was a really simple thing in highmem.c (or 
rather, in include/linux/highmem.h, after kmap_types.h gets included):

#ifndef KM_NMI
# define KM_NMI			-1
#endif
#ifndef KM_NMI_PTE
# define KM_NMI_PTE		-1
#endif

etc.

Thanks,

	Ingo
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