Commit 2e4d8800 authored Mar 02, 2017 by Janosch Frank Committed by Martin Schwidefsky Mar 02, 2017

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: 15f36ebd ("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>

parent 7afbeb6d

arch/s390/mm/pgtable.c

+18 −1

Original line number	Diff line number	Diff line
		@@ -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;

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