linux-kernel - Re: Proposal to realize hot-add *several sections one time*

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Date:	Fri, 13 Jun 2014 15:31:43 +0800
From:	Zhang Zhen <zhenzhang.zhang@...wei.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	<gregkh@...uxfoundation.org>, <laijs@...fujitsu.com>,
	<sjenning@...ux.vnet.ibm.com>, <linux-kernel@...r.kernel.org>,
	<linux-mm@...ck.org>, Wang Nan <wangnan0@...wei.com>
Subject: Re: Proposal to realize hot-add *several sections one time*

On 2014/6/12 15:07, David Rientjes wrote:
> On Thu, 12 Jun 2014, Zhang Zhen wrote:
> 
>>>> % echo start_address_of_new_memory count_of_sections > /sys/devices/system/memory/probe
>>>>
>>>> Then, [start_address_of_new_memory, start_address_of_new_memory +
>>>> count_of_sections * memory_block_size] memory range is hot-added.
>>>>
>>>> If this proposal is reasonable, i will send a patch to realize it.
>>>>
>>>
>>> The problem is knowing how much memory is being onlined so that you can 
>>> definitively determine what count_of_sections should be.  The number of 
>>> pages per memory section depends on PAGE_SIZE and SECTION_SIZE_BITS which 
>>> differ depending on the architectures that support this interface.  So if 
>>> you support count_of_sections, it would return errno even though you have 
>>> onlined some sections.
>>>
>> Hum, sorry.
>> My expression is not right. The count of sections one time hot-added
>> depends on sections_per_block.
>>
> 
> Ok, so you know specifically what sections_per_block is for your platform 
> so you know exactly how many sections need to be added.
> 
>> Now we are porting the memory-hotplug to arm.
>> But we can only hot-add *fixed number of sections one time* on particular architecture.
>>
>> Whether we can add an argument on behalf of the count of the blocks to add ?
>>
>> % echo start_address_of_new_memory count_of_blocks > /sys/devices/system/memory/probe
>>
>> Then, [start_address_of_new_memory, start_address_of_new_memory + count_of_blocks * memory_block_size]
>> memory range is hot-added.
>>
> 
> As I said, if the above returns errno at some point, it still can result 
> in some sections being onlined.  To be clear: if
> "echo 0x10000000 > /sys/devices/system/memory/probe" fails, the section 
> starting at address 0x10000000 failed to be onlined for the reason 
> specified by errno.  If we follow your suggestion to specify how many 
> sections to online, if
> "echo '0x10000000 16' > /sys/devices/system/memory/probe" fails, eight 
> sections could have been successfully onlined at address 0x10000000 and 
> then we encountered a failure (perhaps because the next sections were 
> already onlined, we get an -EEXIST).  We don't know what we successfully 
> onlined.
> 
> This could be mitigated, but there would have to be a convincing reason 
> that this is better than using the currently functionally in a loop and 
> properly handling your error codes.

Hi David,

I think you are right.

We had better to use the currently functionally in a loop if we need to add
several blocks.
In this way, we can get an errno in time if a block failed to be onlined.

Thanks for your comments. I got it.

> 
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