linux-kernel - Re: [PATCH 1/1] xen/balloon: Enforce various limits on target

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Date:	Thu, 7 Mar 2013 12:28:54 +0100
From:	Daniel Kiper <daniel.kiper@...cle.com>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	"carsten@...iers.de" <carsten@...iers.de>,
	"darren.s.shepherd@...il.com" <darren.s.shepherd@...il.com>,
	"james-xen@...gwall.me.uk" <james-xen@...gwall.me.uk>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>
Subject: Re: [PATCH 1/1] xen/balloon: Enforce various limits on target

On Wed, Mar 06, 2013 at 05:52:28PM +0000, David Vrabel wrote:
> On 06/03/13 16:47, Daniel Kiper wrote:

[...]

> >> I think reserve_additional_memory() should check the current resource
> >> map and the e820 map to find a large enough unused region.  This can be
> >> done as an additional patch at a later date.
> >>
> >>> It does not allow balloon driver to execute infinite
> >>> loops when target exceeds limits in other cases too.
> >>
> >> This sentence confuses me.
>
> I'm just confused by the English.  Perhaps it should say:
>
> "The balloon driver will limit target to the maximum reservation as any
> attempt to populate pages above the maximum reservation will always fail."
>
> ?

That is OK.

[...]

> >>> +	if (xen_initial_domain()) {
> >>> +		rc = HYPERVISOR_memory_op(XENMEM_maximum_reservation, &domid);
> >>> +
> >>> +		/* Limit is not enforced by hypervisor. */
> >>> +		if (rc == -EPERM)
> >>> +			goto no_host_limit;
> >>> +
> >>> +		if (rc <= 0) {
> >>> +			pr_info("xen_balloon: %s: Initial domain target limit "
> >>> +				"could not be established: %i\n", __func__, rc);
> >>> +			goto no_host_limit;
> >>> +		}
> >>> +
> >>> +		host_limit = rc;
> >>
> >> I think you should use this method for both dom0 and domUs.  No need to
> >> check static-max from xenstore.
> >
> > Sadly XENMEM_maximum_reservation for domU returns value which is set by xl mem-set
> > not by xl mem-max :-(((... That is why I get this value from xenstore.
>
> It gets d->max_pages which the limit for d->tot_pages. d->max_pages is
> set by xl mem-max (and xl mem-set as it uses the enforce option to

No, it was tested by me and d->max_pages is set by xl mem-set (it is
current target). d->tot_pages has count of pages for a given moment.
It is confusing for me but this is the reality.

> libxl_set_memory_target()).
>
> If you set the target above d->max_pages you won't be able to populate them.
>
> So, using the maximum_reservation call seems like the right thing to me.

Please look above. If you use this value you would not
be able to increase reservation.

Daniel
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