linux-kernel - Re: [PATCH 10/11] x86, mem-hotplug: Support initialize page tables from low to high.

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Message-ID: <52293118.8080707@cn.fujitsu.com>
Date:	Fri, 06 Sep 2013 09:34:16 +0800
From:	Tang Chen <tangchen@...fujitsu.com>
To:	Wanpeng Li <liwanp@...ux.vnet.ibm.com>
CC:	rjw@...k.pl, lenb@...nel.org, tglx@...utronix.de, mingo@...e.hu,
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Subject: Re: [PATCH 10/11] x86, mem-hotplug: Support initialize page tables
 from low to high.

Hi Wanpeng,

Thank you for reviewing. See below, please.

On 09/05/2013 09:30 PM, Wanpeng Li wrote:
......
>> +#ifdef CONFIG_MOVABLE_NODE
>> +	unsigned long kernel_end;
>> +
>> +	if (movablenode_enable_srat&&
>> +	    memblock.current_order == MEMBLOCK_ORDER_LOW_TO_HIGH) {
>
> I think memblock.current_order == MEMBLOCK_ORDER_LOW_TO_HIGH is always
> true if config MOVABLE_NODE and movablenode_enable_srat == true if PATCH
> 11/11 is applied.

memblock.current_order == MEMBLOCK_ORDER_LOW_TO_HIGH is true here if 
MOVABLE_NODE
is configured, and it will be reset after SRAT is parsed. But 
movablenode_enable_srat
could only be true when users specify movablenode boot option in the 
kernel commandline.

Please refer to patch 9/11.

>
>> +		kernel_end = round_up(__pa_symbol(_end), PMD_SIZE);
>> +
>> +		memory_map_from_low(kernel_end, end);
>> +		memory_map_from_low(ISA_END_ADDRESS, kernel_end);
>
> Why split ISA_END_ADDRESS ~ end?

The first 5 pages for the page tables are from brk, please refer to 
alloc_low_pages().
They are able to map about 2MB memory. And this 2MB memory will be used 
to store
page tables for the next mapped pages.

Here, we split [ISA_END_ADDRESS, end) into [ISA_END_ADDRESS, _end) and 
[_end, end),
and map [_end, end) first. This is because memory in [ISA_END_ADDRESS, 
_end) may be
used, then we have not enough memory for the next coming page tables. We 
should map
[_end, end) first because this memory is highly likely unused.

>
......
>
> I think the variables sorted by address is:
> ISA_END_ADDRESS ->  _end ->  real_end ->  end

Yes.

>
>> +	memory_map_from_high(ISA_END_ADDRESS, real_end);
>
> If this is overlap with work done between #ifdef CONFIG_MOVABLE_NODE and
> #endif?
>

I don't think so. Seeing from my code, if work between #ifdef 
CONFIG_MOVABLE_NODE and
#endif is done, it will goto out, right ?

Thanks.

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