s390/mm: limit number of real-space gmap shadows

We have no known user of real-space designation and only support it to
be architecture compliant.

Gmap shadows with real-space designation are never unshadowed
automatically, as there is nothing to protect for the top level table.

So let's simply limit the number of such shadows to one by removing
existing ones on creation of another one.

Acked-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
index c07d64f5cdb5..4a1434bc2f0e 100644
--- a/arch/s390/mm/gmap.c
+++ b/arch/s390/mm/gmap.c
@@ -1455,6 +1455,19 @@ struct gmap *gmap_shadow(struct gmap *parent, unsigned long asce,
                 gmap_free(new);
                 return sg;
         }
+        if (asce & _ASCE_REAL_SPACE) {
+                /* only allow one real-space gmap shadow */
+                list_for_each_entry(sg, &parent->children, list) {
+                        if (sg->orig_asce & _ASCE_REAL_SPACE) {
+                                spin_lock(&sg->guest_table_lock);
+                                gmap_unshadow(sg);
+                                spin_unlock(&sg->guest_table_lock);
+                                list_del(&sg->list);
+                                gmap_put(sg);
+                                break;
+                        }
+                }
+        }
         atomic_set(&new->ref_count, 2);
         list_add(&new->list, &parent->children);
         if (asce & _ASCE_REAL_SPACE) {