Patch "KVM: s390: Fix guest migration for huge guests resulting in panic" has been added to the 4.10-stable tree

This is a note to let you know that I've just added the patch titled

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

to the 4.10-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
kvm-s390-fix-guest-migration-for-huge-guests-resulting-in-panic.patch
and it can be found in the queue-4.10 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Janosch Frank <[email protected]>
Date: Thu, 2 Mar 2017 15:23:42 +0100
Subject: KVM: s390: Fix guest migration for huge guests resulting in panic

From: Janosch Frank <[email protected]>

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.

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

@@ -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;
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.10/kvm-s390-fix-guest-migration-for-huge-guests-resulting-in-panic.patch