Re: [Xen-devel] [PATCH RFC] xen/balloon: check whether a page is pointed to scratch page MFN

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Thu, Sep 05, 2013 at 03:21:32PM +0100, Stefano Stabellini wrote:
> On Thu, 5 Sep 2013, Wei Liu wrote:
> > Now that balloon pages might have two kind of P2M entries, a)
> > INVALID_P2M_ENTRY when that page is never used, b) mfn of one of the
> > many ballooned scratch pages. Refelct this in increase_reservation to
> > avoid hitting BUG_ON.
> > 
> > Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
> > ---
> >  drivers/xen/balloon.c |   21 ++++++++++++++++++++-
> >  1 file changed, 20 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> > index 3101cf6..63a7e5e 100644
> > --- a/drivers/xen/balloon.c
> > +++ b/drivers/xen/balloon.c
> > @@ -305,6 +305,24 @@ static enum bp_state reserve_additional_memory(long 
> > credit)
> >  }
> >  #endif /* CONFIG_XEN_BALLOON_MEMORY_HOTPLUG */
> >  
> > +static bool p2m_is_ballooned_scratch_page(unsigned long pfn)
> > +{
> > +   int cpu;
> > +   unsigned long bpfn;
> > +   struct page *bpage;
> > +
> > +   for_each_possible_cpu(cpu) {
> > +           bpage = per_cpu(balloon_scratch_page, cpu);
> > +           if (!bpage)
> > +                   continue;
> > +           bpfn = page_to_pfn(bpage);
> > +           if (pfn_to_mfn(pfn) == pfn_to_mfn(bpfn))
> 
> Even though it is true that this function doesn't need to be very fast,
> we might as well try to write in an efficient way.
> For example it might be worth calculating pfn_to_mfn(pfn) just once
> outside the loop?
> 

Good point.

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