linux-kernel - Re: [PATCH 1/2] add helper for highmem checks

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Date:	Fri, 08 Feb 2013 12:50:15 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
CC:	linux-kernel@...r.kernel.org, linux-mm@...ck.org, bp@...en8.de,
	mingo@...nel.org, tglx@...utronix.de
Subject: Re: [PATCH 1/2] add helper for highmem checks

On 02/08/2013 12:28 PM, Dave Hansen wrote:
>   #endif
> diff -puN include/linux/mm.h~clean-up-highmem-checks include/linux/mm.h
> --- linux-2.6.git/include/linux/mm.h~clean-up-highmem-checks	2013-02-08 08:42:37.295222148 -0800
> +++ linux-2.6.git-dave/include/linux/mm.h	2013-02-08 09:01:49.758254468 -0800
> @@ -1771,5 +1771,18 @@ static inline unsigned int debug_guardpa
>   static inline bool page_is_guard(struct page *page) { return false; }
>   #endif /* CONFIG_DEBUG_PAGEALLOC */
>
> +static inline phys_addr_t last_lowmem_phys_addr(void)
> +{
> +	/*
> +	 * 'high_memory' is not a pointer that can be dereferenced, so
> +	 * avoid calling __pa() on it directly.
> +	 */
> +	return __pa(high_memory - 1);
> +}
> +static inline bool phys_addr_is_highmem(phys_addr_t addr)
> +{
> +	return addr > last_lowmem_paddr();
> +}
> +

Are we sure that high_memory - 1 is always a valid reference?  Consider 
especially the case where there is MMIO beyond end of memory on a system 
which has less RAM than the HIGHMEM boundary...

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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