Re: [Xen-ia64-devel] [PATCH] fix gmfn_to_mfn

Isaku Yamahata writes:
> On Fri, May 09, 2008 at 03:07:53PM +0900, Kouya Shimura wrote:
> Content-Description: message body text
> 
> > @@ -2838,7 +2842,7 @@ arch_memory_op(int op, XEN_GUEST_HANDLE(
> >              goto out;
> >  
> >          /* Remove previously mapped page if it was present. */
> > -        if (prev_mfn && mfn_valid(prev_mfn)) {
> > +        if (mfn_valid(prev_mfn)) {
> >              if (is_xen_heap_mfn(prev_mfn))
> >                  /* Xen heap frames are simply unhooked from this phys 
> > slot. */
> >                  guest_physmap_remove_page(d, xatp.gpfn, prev_mfn);
> 
> Isn't prev_mfn != 0 check necessary?
> guest_physmap_remove_page() with mfn = 0 hits BUG_ON()
> (and guest_remove_page() calls guest_physmap_remove_page())

Hmm, Should BUG_ON() be removed from guest_physmap_remove_page()?
If the page(mfn=0) is allocated to a guest, the guest can crash xen.

prev_mfn was once 0 when the page was I/O page until cset
17316:408fcc50fd35. So I think this check is used to skip removing
the I/O page.

FYI, on x86 side, prev_mfn != 0 is not checked.

Thanks,
Kouya

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