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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Apr 09, 2014 at 03:21:38PM -0700, Matthew Rushton wrote:
> 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@xxxxxxxxxx> wrote:
> >>>On Wed, 2014-04-02 at 08:52 +0100, Jan Beulich wrote:
> >>>>>>>On 02.04.14 at 02:17, <mvrushton@xxxxxxxxx> 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?

I think Tim was OK with that - as long as it was based on a flag - meaning
when we do the increase_reservation call we use an extra flag
to ask for contingous PFNs.

> 
> >Ian.
> >
> 
> 
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