kunmap_atomic() must zap the PTE to avoid dangling references

when attempting to free memory back to Xen. We can implement
something more efficient in future.

Also add debug print message if guest tries to free 'in use'
memory. We'll make it a real guest-visible error in future.

Signed-off-by: Keir Fraser <keir@xensource.com>

linux-2.6-xen-sparse/arch/i386/mm/highmem-xen.c

@@ -79,6 +79,16 @@ void kunmap_atomic(void *kvaddr, enum km_type type)
 	 */
 	pte_clear(&init_mm, vaddr, kmap_pte-idx);
 	__flush_tlb_one(vaddr);
+#elif defined(CONFIG_XEN)
+	/*
+	 * We must ensure there are no dangling pagetable references when
+	 * returning memory to Xen (decrease_reservation).
+	 * XXX TODO: We could make this faster by only zapping when
+	 * kmap_flush_unused is called but that is trickier and more invasive.
+	 */
+	unsigned long vaddr = (unsigned long) kvaddr & PAGE_MASK;
+	enum fixed_addresses idx = type + KM_TYPE_NR*smp_processor_id();
+	pte_clear(&init_mm, vaddr, kmap_pte-idx);
 #endif
 
 	dec_preempt_count();

xen/common/memory.c

@@ -170,6 +170,15 @@ guest_remove_page(
     if ( test_and_clear_bit(_PGC_allocated, &page->count_info) )
         put_page(page);
 
+    if ( unlikely((page->count_info & PGC_count_mask) != 1) )
+    {
+        /* We'll make this a guest-visible error in future, so take heed! */
+        DPRINTK("Dom%d freeing in-use page %lx (pseudophys %lx):"
+                " count=%x type=%lx\n",
+                d->domain_id, mfn, get_gpfn_from_mfn(mfn),
+                page->count_info, page->u.inuse.type_info);
+    }
+
     guest_physmap_remove_page(d, gmfn, mfn);
 
     put_page(page);