[PATCH 4.10 41/48] KVM: s390: Fix guest migration for huge guests resulting in panic

Greg Kroah-Hartman
2017-03-16T14:36:03Z

From: Janosch Frank <frankja@linux.vnet.ibm.com>

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.")
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>

---
arch/s390/mm/pgtable.c | 19 ++++++++++++++++++-
1 file changed, 18 insertions(+), 1 deletion(-)

--- a/arch/s390/mm/pgtable.c
+++ b/arch/s390/mm/pgtable.c
@@ -606,12 +606,29 @@ void ptep_zap_key(struct mm_struct *mm,
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;