Re: [PATCH v4 1/2] memory-hotplug: add automatic onlining policy for the newly added memory

[Vitaly Kuznetsov <vkuznets@redhat.com>]

David Rientjes <rientjes@google.com> writes:

> On Tue, 12 Jan 2016, Vitaly Kuznetsov wrote:
>
>> diff --git a/Documentation/memory-hotplug.txt b/Documentation/memory-hotplug.txt
>> index ce2cfcf..ceaf40c 100644
>> --- a/Documentation/memory-hotplug.txt
>> +++ b/Documentation/memory-hotplug.txt
>> @@ -254,12 +254,23 @@ If the memory block is online, you'll read "online".
>>  If the memory block is offline, you'll read "offline".
>>  
>>  
>> -5.2. How to online memory
>> +5.2. Memory onlining
>
> Idk why you're changing this title since you didn't change it in the table 
> of contents and it already pairs with "6.2. How to offline memory".
>
> This makes it seem like you're covering all memory onlining operations in 
> the kernel (including xen onlining) rather than just memory onlined by 
> root.  It doesn't cover the fact that xen onlining can be done without 
> automatic onlining, so I would leave this section's title as it is and 
> only cover aspects of memory onlining that users are triggering 
> themselves.

Ok, I changed the title to reflect the fact that a special action to
online memory is not always required any more but as the global policy
stays 'offline' by default for now let's keep the original title.

[skip]

>>  
>> +	/* online pages if requested */
>> +	if (online)
>> +		online_pages(start >> PAGE_SHIFT, size >> PAGE_SHIFT,
>> +			     MMOP_ONLINE_KEEP);
>> +
>>  	goto out;
>>  
>>  error:
>
> Well, shucks, what happens if online_pages() fails, such as if a memory 
> hot-add notifier returns an errno for MEMORY_GOING_ONLINE?  The memory was 
> added but not subsequently onlined, although auto onlining was set, so how 
> does userspace know the state it is in?

Bad ... we could have checked the return value but I don't see a proper
way to handling it here: if we managed to online some blocks we can't
revert back. We'll probably have to online pages block-by-block (e.g. by
utilizing memory_block_change_state()) handling possible failures.

>
>> @@ -1315,7 +1327,7 @@ int __ref add_memory(int nid, u64 start, u64 size)
>>  	if (!res)
>>  		return -EEXIST;
>>  
>> -	ret = add_memory_resource(nid, res);
>> +	ret = add_memory_resource(nid, res, memhp_auto_online);
>>  	if (ret < 0)
>>  		release_memory_resource(res);
>>  	return ret;

-- 
  Vitaly

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