Re: [PATCH part1 v6 4/6] x86/mem-hotplug: Support initialize page tables in bottom-up

[Zhang Yanfei <zhangyanfei.yes@gmail.com>]

Hello peter,

On 10/07/2013 08:00 AM, H. Peter Anvin wrote:
> On 10/03/2013 07:00 PM, Zhang Yanfei wrote:
>> From: Tang Chen <tangchen@cn.fujitsu.com>
>>
>> The Linux kernel cannot migrate pages used by the kernel. As a
>> result, kernel pages cannot be hot-removed. So we cannot allocate
>> hotpluggable memory for the kernel.
>>
>> In a memory hotplug system, any numa node the kernel resides in
>> should be unhotpluggable. And for a modern server, each node could
>> have at least 16GB memory. So memory around the kernel image is
>> highly likely unhotpluggable.
>>
>> ACPI SRAT (System Resource Affinity Table) contains the memory
>> hotplug info. But before SRAT is parsed, memblock has already
>> started to allocate memory for the kernel. So we need to prevent
>> memblock from doing this.
>>
>> So direct memory mapping page tables setup is the case. init_mem_mapping()
>> is called before SRAT is parsed. To prevent page tables being allocated
>> within hotpluggable memory, we will use bottom-up direction to allocate
>> page tables from the end of kernel image to the higher memory.
>>
>> Acked-by: Tejun Heo <tj@kernel.org>
>> Signed-off-by: Tang Chen <tangchen@cn.fujitsu.com>
>> Signed-off-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
> 
> I'm still seriously concerned about this.  This unconditionally
> introduces new behavior which may very well break some classes of

Well, this new behaviour is not unconditional, if user doesn't specify
the movable_node option, the kernel will act as before, allocating
memory top-down.

> systems -- the whole point of creating the page tables top down is
> because the kernel tends to be allocated in lower memory, which is also
> the memory that some devices need for DMA.

How much memory does these devices needed for DMA? And you mean memory
under 16MB or 4GB?

> 
> +#ifdef CONFIG_X86
> +		kernel_end = __pa_symbol(_end);
> +#else
> +		kernel_end = __pa(RELOC_HIDE((unsigned long)(_end), 0));
> +#endif
> 
> We really should make __pa_symbol() available everywhere by putting
> something like the above in a global define (under #ifndef __pa_symbol).

Hmmmm...in include/asm-generic/page.h?

> 
> Is RELOC_HIDE() even correct here?

Sorry, could you explain a bit?

-- 
Thanks.
Zhang Yanfei

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