Re: mem_sharing: summarized problems when domain is dying

[George Dunlap <George.Dunlap@eu.citrix.com>]

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Tim / Xiaoyun, do you think something like this might work?

 -George

On Fri, Jan 21, 2011 at 4:32 PM, George Dunlap <dunlapg@umich.edu> wrote:
> [sorry, accidentally sent too early]
>
> On Fri, Jan 21, 2011 at 4:29 PM, George Dunlap <dunlapg@umich.edu> wrote:
>> I'm not sure the exact rationale behind the "cannot fail" flag; but it
>> looks like in grant_table.c, both callers of gfn_to_mfn_unshare()
>> handle the case where the returned p2m entry is just
>
> ...invalid.  I wonder if "unsharing" the page, but marking the entry
> invalid during death would help.
>
> I suppose the problem there is that if you're keeping the VM around
> but paused for analysis, you'll have holes in your address space.  But
> just returning an invalid entry to the callers who try to unshare
> pages might work.
>
>  -George
>

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diff -r 9ca9331c9780 xen/include/asm-x86/p2m.h
--- a/xen/include/asm-x86/p2m.h	Fri Jan 21 15:37:36 2011 +0000
+++ b/xen/include/asm-x86/p2m.h	Fri Jan 21 16:41:58 2011 +0000
@@ -390,7 +390,14 @@
                                       must_succeed 
                                       ? MEM_SHARING_MUST_SUCCEED : 0) )
         {
-            BUG_ON(must_succeed);
+            if ( must_succeed
+                 && p2m->domain->is_dying )
+            {
+                mfn = INVALID_MFN;
+                *p2mt=p2m_invalid;
+            }
+            else
+                BUG_ON(must_succeed);
             return mfn;
         }
         mfn = gfn_to_mfn(p2m, gfn, p2mt);

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