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Subject[PATCH 3.18 125/145] KVM: s390: Fix guest migration for huge guests resulting in panic
3.18-stable review patch.  If anyone has any objections, please let me know.


From: Janosch Frank <>

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 <>
Acked-by: Christian Borntraeger <>
Signed-off-by: Martin Schwidefsky <>
Signed-off-by: Greg Kroah-Hartman <>
Signed-off-by: Greg Kroah-Hartman <>

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
@@ -1358,11 +1358,28 @@ EXPORT_SYMBOL_GPL(s390_enable_skey);
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;

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