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

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>

1 files changed, 18 insertions, 1 deletions

diff --git a/arch/s390/mm/pgtable.c b/arch/s390/mm/pgtable.c
index b48dc5f1900b..463e5ef02304 100644
--- a/arch/s390/mm/pgtable.c
+++ b/arch/s390/mm/pgtable.c
@@ -608,12 +608,29 @@ void ptep_zap_key(struct mm_struct *mm, unsigned long addr, pte_t *ptep)
 bool test_and_clear_guest_dirty(struct mm_struct *mm, unsigned long addr)
 {
         spinlock_t *ptl;
+        pgd_t *pgd;
+        pud_t *pud;
+        pmd_t *pmd;
         pgste_t pgste;
         pte_t *ptep;
         pte_t pte;
         bool dirty;
 
-        ptep = get_locked_pte(mm, addr, &ptl);
+        pgd = pgd_offset(mm, addr);
+        pud = pud_alloc(mm, pgd, addr);
+        if (!pud)
+                return false;
+        pmd = pmd_alloc(mm, pud, addr);
+        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;
+
+        ptep = pte_alloc_map_lock(mm, pmd, addr, &ptl);
         if (unlikely(!ptep))
                 return false;