Re: [Qemu-devel] [RFC PATCH v2 2/2] spapr: Memory hot-unplug support

[Thomas Huth]

On 27.04.2016 15:37, Igor Mammedov wrote:
> On Tue, 26 Apr 2016 16:03:37 -0500
> Michael Roth <address@hidden> wrote:
> 
>> Quoting Igor Mammedov (2016-04-26 02:52:36)
>>> On Tue, 26 Apr 2016 10:39:23 +0530
>>> Bharata B Rao <address@hidden> wrote:
>>>   
>>>> On Mon, Apr 25, 2016 at 11:20:50AM +0200, Igor Mammedov wrote:  
>>>>> On Wed, 16 Mar 2016 10:11:54 +0530
>>>>> Bharata B Rao <address@hidden> wrote:
>>>>>     
>>>>>> On Wed, Mar 16, 2016 at 12:36:05PM +1100, David Gibson wrote:    
>>>>>>> On Tue, Mar 15, 2016 at 10:08:56AM +0530, Bharata B Rao wrote:      
>>>>>>>> Add support to hot remove pc-dimm memory devices.
>>>>>>>>
>>>>>>>> Signed-off-by: Bharata B Rao <address@hidden>      
>>>>>>>
>>>>>>> Reviewed-by: David Gibson <address@hidden>
>>>>>>>
>>>>>>> Looks correct, but again, needs to wait on the PAPR change.    
>>>>> [...]    
>>>>>>
>>>>>> While we are here, I would also like to get some opinion on the real
>>>>>> need for memory unplug. Is there anything that memory unplug gives us
>>>>>> which memory ballooning (shrinking mem via ballooning) can't give ?    
>>>>> Sure ballooning can complement memory hotplug but turning it on would
>>>>> effectively reduce hotplug to balloning as it would enable overcommit
>>>>> capability instead of hard partitioning pc-dimms provides. So one
>>>>> could just use ballooning only and not bother with hotplug at all.
>>>>>
>>>>> On the other hand memory hotplug/unplug (at least on x86) tries
>>>>> to model real hardware, thus removing need in paravirt ballooning
>>>>> solution in favor of native guest support.    
>>>>
>>>> Thanks for your views.
>>>>   
>>>>>
>>>>> PS:
>>>>> Guest wise, currently hot-unplug is not well supported in linux,
>>>>> i.e. it's not guarantied that guest will honor unplug request
>>>>> as it may pin dimm by using it as a non migratable memory. So
>>>>> there is something to work on guest side to make unplug more
>>>>> reliable/guarantied.    
>>>>
>>>> In the above scenario where the guest doesn't allow removal of certain
>>>> parts of DIMM memory, what is the expected behaviour as far as QEMU
>>>> DIMM device is concerned ? I seem to be running into this situation
>>>> very often with PowerPC mem unplug where I am left with a DIMM device
>>>> that has only some memory blocks released. In this situation, I would like
>>>> to block further unplug requests on the same device, but QEMU seems
>>>> to allow more such unplug requests to come in via the monitor. So
>>>> qdev won't help me here ? Should I detect such condition from the
>>>> machine unplug() handler and take required action ?  
>>> I think offlining is a guests task along with recovering from
>>> inability to offline (i.e. offline all + eject or restore original state).
>>> QUEM does it's job by notifying guest what dimm it wants to remove
>>> and removes it when guest asks it (at least in x86 world).  
>>
>> In the case of pseries, the DIMM abstraction isn't really exposed to
>> the guest, but rather the memory blocks we use to make the backing
>> memdev memory available to the guest. During unplug, the guest
>> completely releases these blocks back to QEMU, and if it can only
>> release a subset of what's requested it does not attempt to recover.
>> We can potentially change that behavior on the guest side, since
>> partially-freed DIMMs aren't currently useful on the host-side...
>>
>> But, in the case of pseries, I wonder if it makes sense to maybe go
>> ahead and MADV_DONTNEED the ranges backing these released blocks so the
>> host can at least partially reclaim the memory from a partially
>> unplugged DIMM?
> It's a little bit confusing, one asked to remove device but it's still
> there but not completely usable/available.
> What will happen when user wants that memory plugged back?

As far as I've understood MADV_DONTNEED, you can use the memory again at
any time - just the previous contents will be gone, which is ok in this
case since the guest previously marked this area as unavailable.

 Thomas