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

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Date:	Fri, 08 Feb 2013 15:16:18 -0800
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	"H. Peter Anvin" <hpa@...or.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 12:50 PM, H. Peter Anvin wrote:
> On 02/08/2013 12:28 PM, Dave Hansen wrote:
>> +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...

Yeah, I think it is.  "high_memory" should point at either the end of
RAM, or the end of the linear map, whichever is lower.  See setup_arch():

        max_pfn = e820_end_of_ram_pfn();
	...
        high_memory = (void *)__va(max_pfn * PAGE_SIZE - 1) + 1;

or in the highmem init code:

        high_memory = (void *) __va(max_low_pfn * PAGE_SIZE - 1) + 1;

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