[PATCH] mem_sharing_nominate_page: p2mt should never change before p2m_change_type()

[Nai Xia <nai.xia@gmail.com>]

The p2mt change check for p2m_change_type() was first introduced when this code path was not protected by p2m_lock().
Now this code path is protected by p2m_lock. So p2mt should never change before p2m_change_type().

---
diff --git a/xen/arch/x86/mm/mem_sharing.c b/xen/arch/x86/mm/mem_sharing.c
index 0364bb0..b3940cb 100644
--- a/xen/arch/x86/mm/mem_sharing.c
+++ b/xen/arch/x86/mm/mem_sharing.c
@@ -900,20 +900,8 @@ int mem_sharing_nominate_page(struct domain *d,
         goto out;
     }

-    /* Change the p2m type */
-    if ( p2m_change_type(d, gfn, p2mt, p2m_ram_shared) != p2mt )
-    {
-        /* This is unlikely, as the type must have changed since we've checked
-         * it a few lines above.
-         * The mfn needs to revert back to rw type. This should never fail,
-         * since no-one knew that the mfn was temporarily sharable */
-        mem_sharing_gfn_destroy(page, d, gfn_info);
-        xfree(page->sharing);
-        page->sharing = NULL;
-        /* NOTE: We haven't yet added this to the audit list. */
-        BUG_ON(page_make_private(d, page) != 0);
-        goto out;
-    }
+    /* Change the p2m type, should never fail with p2m locked. */
+    BUG_ON(p2m_change_type(d, gfn, p2mt, p2m_ram_shared) != p2mt);

     /* Account for this page. */
     atomic_inc(&nr_shared_mfns);