Re: [Xen-devel] [PATCH] xen: set P2M entry to INVALID_P2M_ENTRY for ballooned out pages

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Aug 27, 2013 at 02:34:42PM +0100, Stefano Stabellini wrote:
> On Mon, 26 Aug 2013, Wei Liu wrote:
> > On Thu, Aug 22, 2013 at 02:00:05PM +0100, David Vrabel wrote:
> > > On 22/08/13 13:57, Wei Liu wrote:
> > > > In commit cd9151e2: xen/balloon: set a mapping for ballooned out pages
> > > > we have the ballooned out page's mapping set to a scratch page. That 
> > > > commit
> > > > also set the P2M entry of ballooned out page to the scratch, which is
> > > > not necessary. That triggers BUG_ONs in __set_phys_to_machine when the
> > > > page is ballooned in again.
> > > > 
> > > > We only need to ensure that the ballooned out pages have valid mapping.
> > > > P2M entries of those pages should still be set to INVALID_P2M_ENTRY.
> > > 
> > > I know I suggested this but I would like to get Stefano's acked-by or
> > > reviewed-by for this.
> > > 
> > 
> > Stefano, what do you think?
> 
> I think it is wrong: if the page has a valid mapping it should also have
> a valid p2m entry. It's easier to think about and it's going to avoid
> bugs when generic linux code ends up calling a pvop that on Xen looks
> into the p2m.
> 
> Which one of the BUG_ON in __set_phys_to_machine are hit?  We could

Both.

> avoid calling set_phys_to_machine in decrease_reservation on
> XENFEAT_auto_translated_physmap guests, that would avoid the first
> BUG_ON in __set_phys_to_machine. Arguably it's the right thing to do
> anyway.
> 

OK.

> Regarding the second BUG_ON:
> 
> if (unlikely(pfn >= MAX_P2M_PFN)) {
>     BUG_ON(mfn != INVALID_P2M_ENTRY);
>     return true;
> }
> 
> We shouldn't really be hitting it, right? Why is pfn >= MAX_P2M_PFN?

I think I did something silly, but I could only vaguely remember that I
tried to play with very high virtual address, which could lead to pfn >=
MAX_P2M_PFN.

AIUI we can definitively hit this case otherwise we would just have

 BUG_ON(pfn >= MAX_P2M_PFN)

right?

Wei.

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