Re: [Xen-devel] Balloon driver bug in increase_reservation

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Mon, 2 Sep 2013, Ian Campbell wrote:
> On Mon, 2013-09-02 at 16:13 +0100, Wei Liu wrote:
> > On Mon, Sep 02, 2013 at 04:09:26PM +0100, Ian Campbell wrote:
> > > On Mon, 2013-09-02 at 16:04 +0100, Wei Liu wrote:
> > > > On Mon, Sep 02, 2013 at 03:48:43PM +0100, Ian Campbell wrote:
> > > > > On Mon, 2013-09-02 at 15:43 +0100, Wei Liu wrote:
> > > > > > Hi, Stefano
> > > > > > 
> > > > > > I found another bug in the balloon scratch page code. As I didn't 
> > > > > > follow
> > > > > > the discussion on scratch page so I cannot propose a proper fix at 
> > > > > > the
> > > > > > moment.
> > > > > > 
> > > > > > The problem is that in balloon.c:increase_reservation, when a 
> > > > > > ballooned
> > > > > > page is resued, it can have a valid P2M entry pointing to the 
> > > > > > scratch,
> > > > > > hitting the BUG_ON
> > > > > > 
> > > > > >    BUG_ON(!xen_feature(XENFEAT_auto_translated_physmap) &&
> > > > > >           phys_to_machine_mapping_valid(pfn));
> > > > > > 
> > > > > > As balloon worker might run by a CPU other then the one that 
> > > > > > returns the
> > > > > > page, checking pfn_to_mfn(pfn) == local_cpu_scratch_page_mfn 
> > > > > > wouldn't
> > > > > > work.  Checking pfn_to_mfn(pfn) belongs to the set of all scratch 
> > > > > > page
> > > > > > mfns is not desirable.
> > > > > 
> > > > > This makes me think that whoever suggested that pfn_to_mfn for a
> > > > > ballooned page out to return INVALID_MFN was right.
> > > > > 
> > > > 
> > > > If there are many balloon pages the check can be expensive.
> > > 
> > > IIRC the suggestion was that the p2m for a ballooned out page would
> > > contain INVALID_MFN, so the expense is just the lookup you would be
> > > doing anyway.
> > > 
> > 
> > That was David's idea. Stefano was worried that other PVOPS hooks would
> > need to know about the MFN. I don't know how much it holds true. Need
> > some input from Konrad I think.
> 
> Hrm, aren't those expected to not be operating on ballooned out pages
> anyway? Prior to this change those callers would have got INVALID_MFN, I
> think?
> 
> My gut feeling is that places which accidentally/unexpectedly have a
> ballooned out page in their hand have either a virtual address or an mfn
> (e.g. a cr2 fault address) and not a pfn in their hands, and won't in
> general be that interested in the pfn and/or are already geared up to
> deal with INVALID_MFN because previously that's what they would have
> gotten. There's every chance I'm wrong though.


I think we should just remove the BUG_ON:

- if the guest is autotranslate, then the BUG_ON is already irrelevant;
- if the guest is not autotranslate, then we are sure that the p2m of
  the page is pointing to a scratch page;

either way the BUG_ON is wrong.

Otherwise could simply implement a is_balloon_scratch_page function that
checks whether a given pfn corresponds to any of the scratch pages (it
doesn't need to be the scratch page of this cpu).

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