KVM: s390: Fix guest migration for huge guests resulting in panic

commit 2e4d88009f57057df7672fa69a32b5224af54d37 upstream.

While we can technically not run huge page guests right now, we can
setup a guest with huge pages. Trying to migrate it will trigger a
VM_BUG_ON and, if the kernel is not configured to panic on a BUG, it
will happily try to work on non-existing page table entries.

With this patch, we always return "dirty" if we encounter a large page
when migrating. This at least fixes the immediate problem until we
have proper handling for both kind of pages.

Fixes: 15f36eb ("KVM: s390: Add proper dirty bitmap support to S390 kvm.")
Cc: <stable@vger.kernel.org> # 3.16+

Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Janosch Frank

arch/s390/mm/pgtable.c

@@ -1237,11 +1237,28 @@ EXPORT_SYMBOL_GPL(s390_reset_cmma);
  */
 bool gmap_test_and_clear_dirty(unsigned long address, struct gmap *gmap)
 {
+	pgd_t *pgd;
+	pud_t *pud;
+	pmd_t *pmd;
 	pte_t *pte;
 	spinlock_t *ptl;
 	bool dirty = false;
 
-	pte = get_locked_pte(gmap->mm, address, &ptl);
+	pgd = pgd_offset(gmap->mm, address);
+	pud = pud_alloc(gmap->mm, pgd, address);
+	if (!pud)
+		return false;
+	pmd = pmd_alloc(gmap->mm, pud, address);
+	if (!pmd)
+		return false;
+	/* We can't run guests backed by huge pages, but userspace can
+	 * still set them up and then try to migrate them without any
+	 * migration support.
+	 */
+	if (pmd_large(*pmd))
+		return true;
+
+	pte = pte_alloc_map_lock(gmap->mm, pmd, address, &ptl);
 	if (unlikely(!pte))
 		return false;