KVM: Minor memory slot optimization

If a slot is removed or moved in the guest physical address space, we
first allocate and install a new slot array with the invalidated
entry.  The old array is then freed.  We then proceed to allocate yet
another slot array to install the permanent replacement.  Re-use the
original array when this occurs and avoid the extra kfree/kmalloc.
Reviewed-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

virt/kvm/kvm_main.c

@@ -716,7 +716,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
 	unsigned long npages;
 	struct kvm_memory_slot *memslot, *slot;
 	struct kvm_memory_slot old, new;
-	struct kvm_memslots *slots, *old_memslots;
+	struct kvm_memslots *slots = NULL, *old_memslots;
 
 	r = check_memory_region_flags(mem);
 	if (r)
@@ -832,18 +832,25 @@ int __kvm_set_memory_region(struct kvm *kvm,
 		 * 	- kvm_is_visible_gfn (mmu_check_roots)
 		 */
 		kvm_arch_flush_shadow_memslot(kvm, slot);
-		kfree(old_memslots);
+		slots = old_memslots;
 	}
 
 	r = kvm_arch_prepare_memory_region(kvm, &new, old, mem, user_alloc);
 	if (r)
-		goto out_free;
+		goto out_slots;
 
 	r = -ENOMEM;
+	/*
+	 * We can re-use the old_memslots from above, the only difference
+	 * from the currently installed memslots is the invalid flag.  This
+	 * will get overwritten by update_memslots anyway.
+	 */
+	if (!slots) {
 		slots = kmemdup(kvm->memslots, sizeof(struct kvm_memslots),
 				GFP_KERNEL);
 		if (!slots)
 			goto out_free;
+	}
 
 	/* map new memory slot into the iommu */
 	if (npages) {