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

[Tim Deegan <tim@xxxxxxx>]

At 06:48 -0400 on 01 Apr (1396331306), Konrad Rzeszutek Wilk wrote:
> On Mon, Mar 31, 2014 at 08:25:43PM -0700, Matthew Rushton wrote:
> > Are there any concerns about the proposed Xen change as a reasonable
> > work around for the current implementation? Thank you!
> 
> So I finally understood what the concern was about it - the balloon
> mechanics get the pages in worst possible order. I am wondeirng if there
> is something on the Linux side we can do to tell Xen to give them to use
> in the proper order?

The best way, at least from Xen's point of view, is to explicitly
allocate contiguous pages in the cases where it'll make a difference
AIUI linux already does this for some classes of dma-able memory.

> Could we swap the order of xen_do_chunk so it starts from the end and
> goes to start? Would that help?

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?

Cheers,

Tim.

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