Re: [RFC PATCH] page_alloc: use first half of higher order chunks when halving

[Matthew Rushton <mvrushton@gmail.com>]

On 04/02/14 03:20, Ian Campbell wrote:
> On Wed, 2014-04-02 at 11:15 +0100, Jan Beulich wrote:
>>>>> On 02.04.14 at 12:06, <Ian.Campbell@citrix.com> wrote:
>>> On Wed, 2014-04-02 at 08:52 +0100, Jan Beulich wrote:
>>>>>>> On 02.04.14 at 02:17, <mvrushton@gmail.com> wrote:
>>>>> On 04/01/14 05:22, Tim Deegan wrote:
>>>>>> As long as we don't also change the default allocation order in
>>>>>> Xen. :)  In general, linux shouldn't rely on the order that Xen
>>>>>> allocates memory, as that might change later.  If the current API
>>>>>> can't do what's needed, maybe we can add another allocator
>>>>>> hypercall or flag?
>>>>> Agree on not relying on the order in the long run. A new hypercall or
>>>>> flag seems like overkill right now. The question for me comes down to my
>>>>> proposed change which is more simple and solves the short term problem
>>>>> or investing time in reworking the Linux code to make large allocations.
>>>> I think it has become pretty clear by now that we'd rather not alter
>>>> the hypervisor allocator for a purpose like this.

OK understood see below.

>>> Does it even actually solve the problem? It seems like it is just
>>> deferring it until sufficient fragmentation has occurred in the system.
>>> All its really done is make the eventual issue much harder to debug.
>> Wasn't this largely for Dom0 (in which case fragmentation shouldn't
>> matter yet)?
> Dom0 ballooning breaks any assumptions you might make about relying on
> early allocations.

I think you're missing the point. I'm not arguing that this change is a 
general purpose solution to guarantee that dom0 is contiguous. 
Fragmentation can exist even if dom0 asks for larger allocations like it 
should (which the balloon driver does I believe). What the change does 
do is solve a real problem in the current Linux PCI remapping 
implementation which happens during dom0 intialization. If the 
allocation strategy is arbitrary why not make the proposed hypervisor 
change to make existing Linux implementations behave better and in 
addition fix the problem in Linux so moving forward things are safe?

> Ian.
>