Re: [RFC 1/2] xen/mm: Clarify the granularity for each Frame Number

[Julien Grall <julien.grall@citrix.com>]

On 12/08/15 12:58, Jan Beulich wrote:
>>>> On 12.08.15 at 13:13, <julien.grall@citrix.com> wrote:
>> On 12/08/15 11:33, Jan Beulich wrote:
>>>>>> On 12.08.15 at 11:57, <julien.grall@citrix.com> wrote:
>>>> On 12/08/2015 08:16, Jan Beulich wrote:
>>>>>>>> On 05.08.15 at 15:18, <julien.grall@citrix.com> wrote:
>>>>>> When the backend is 64K, it will map the foreign 4K at the top of a 64K
>>>>>> page. It's a waste of memory, but it's easier to implement and it's
>>>>>> still and improvement compare to have Linux crashing at boot.
>>>>>
>>>>> Waste of memory? You're only mapping an existing chunk of memory.
>>>>> DYM waste of address space?
>>>>
>>>> No, I really meant waste of memory. The current grant API in Linux is 
>>>> allocating one Linux Page per grant. Although the grant is always 4K, so 
>>>> we won't be able to make use of the 60K for anything as long as we use 
>>>> this page for a grant.
>>>>
>>>> So if the grant is pre-allocated (such as for PV block), we won't be 
>>>> able use nr_grant * 60KB memory.
>>>
>>> I still don't follow - grant mappings ought to be done into ballooned
>>> (i.e. empty) pages, i.e. no memory would get wasted unless there
>>> are too few balloon pages available.
>>
>> Everything balloon out is less memory that can be used by Linux. If we
>> are only using 1/15 of the balloon out page that a huge waste of memory
>> for me.
> 
> I still don't get it: A ballooned page does not represent any memory
> (and at least on x86 iirc we start PV domains with 8Mb sort-of-
> pre-ballooned space). Where is the loss you're talking about coming
> from? Or, wait - you don't have PoD (yet) on ARM, so you have no
> choice.

I'm not sure how PoD would affect the problem here...

An ARM guest can be considered as x86 HVM guest + PV drivers. At domain
creation, all the RAM is allocated for this domain.

Furthermore we have no memory hotplug support, so we would steal RAM
page every time we have to balloon out a page.

Regards,

-- 
Julien Grall