linux-kernel - Re: [tip:perfcounters/core] x86: Add NMI types for kmap

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Date:	Mon, 15 Jun 2009 16:30:51 +0100 (BST)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
cc:	linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
	paulus@...ba.org, acme@...hat.com, efault@....de, npiggin@...e.de,
	tglx@...utronix.de, mingo@...e.hu,
	linux-tip-commits@...r.kernel.org,
	"hugh.dickins" <hugh.dickins@...cali.co.uk>
Subject: Re: [tip:perfcounters/core] x86: Add NMI types for kmap_atomic

On Mon, 15 Jun 2009, Peter Zijlstra wrote:
> 
> The below would fix it, but that's getting rather ugly :-/,
> alternatively I would have to introduce something like
> pte_offset_map_irq() which would make the irq/nmi detection and leave
> the regular code paths alone, however that would mean either duplicating
> the gup_fast() pagewalk or passing down a pte function pointer, which
> would only duplicate the gup_pte_range() bit, neither is really
> attractive...

> Index: linux-2.6/arch/x86/include/asm/pgtable_32.h
> ===================================================================
> --- linux-2.6.orig/arch/x86/include/asm/pgtable_32.h
> +++ linux-2.6/arch/x86/include/asm/pgtable_32.h
> @@ -49,7 +49,10 @@ extern void set_pmd_pfn(unsigned long, u
>  #endif
>  
>  #if defined(CONFIG_HIGHPTE)
> -#define __KM_PTE	(in_nmi() ? KM_NMI_PTE : KM_PTE0)
> +#define __KM_PTE			\
> +	(in_nmi() ? KM_NMI_PTE : 	\
> +	 in_irq() ? KM_IRQ_PTE :	\
> +	 KM_PTE0)
>  #define pte_offset_map(dir, address)					\
>  	((pte_t *)kmap_atomic_pte(pmd_page(*(dir)), __KM_PTE) +		\
>  	 pte_index((address)))

Yes, that does look ugly!

I've not been following the background to this, but I've often
wondered if a kmap_push() and kmap_pop() could be useful,
allowing you to reuse the slot in between - any use here?

Hugh
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