[Xen-devel] Re: [PATCH 3/3] xen: use maximum reservation to limit dom0 memory

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

On Tue, Aug 16, 2011 at 11:00:38AM +0100, David Vrabel wrote:
> Use the maximum reservation hypercall to set limit the amount of
> usable dom0 memory.  This reduces the size of pages tables etc. if
> dom0 is to use less memory than the maximum available.

Ok, so it sounds like this patch by itself can fix the "more page tables
than we need" issue.

If so, I would prefer that you stick the tiny piece of code that
calls the xen_get_max_pages() from the setup in this patch. This way
we can backport this particular patch to stable tree without including
the other patchsets you have posted. And it is a nicely contained
one-patch-fixes-the-problem.

> 
> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
> ---
> Note this requires a patched Xen that sets max_pages when creating dom0.

Please mention in the description the c/s and the name of the patch.

> ---
>  arch/x86/xen/setup.c |    7 ++++++-
>  1 files changed, 6 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
> index 3421c9e..584e7dc 100644
> --- a/arch/x86/xen/setup.c
> +++ b/arch/x86/xen/setup.c
> @@ -197,7 +197,12 @@ static unsigned long __init xen_get_max_pages(void)
>       unsigned long max_pages = MAX_DOMAIN_PAGES; /* Limited by memory map. */
>  
>       if (xen_initial_domain()) {
> -             /* FIXME: ask hypervisor for max pages. */
> +             domid_t domid = DOMID_SELF;
> +             int ret;
> +
> +             ret = HYPERVISOR_memory_op(XENMEM_maximum_reservation, &domid);
> +             if (ret > 0)
> +                     max_pages = ret;
Don't you want to clamp it? Say MAX_DOMAIN_PAGES is set to 1GB, and you
set it to 2GB here - that will blow the P2M out. Perhaps
        max_pages = min(ret, max_pages); ?

>       }
>  
>       return min(max_pages, MAX_DOMAIN_PAGES);
> -- 
> 1.7.4.1

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