mm: disable preemption in apply_to_pte_range

Impact: bugfix

Lazy mmu mode needs preemption disabled, so if we're apply to
init_mm (which doesn't require any pte locks), then explicitly
disable preemption.  (Do it unconditionally after checking we've
successfully done the allocation to simplify the error handling.)
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

mm/memory.c

@@ -1718,6 +1718,7 @@ static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
 
 	BUG_ON(pmd_huge(*pmd));
 
+	preempt_disable();
 	arch_enter_lazy_mmu_mode();
 
 	token = pmd_pgtable(*pmd);
@@ -1729,6 +1730,7 @@ static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
 	} while (pte++, addr += PAGE_SIZE, addr != end);
 
 	arch_leave_lazy_mmu_mode();
+	preempt_enable();
 
 	if (mm != &init_mm)
 		pte_unmap_unlock(pte-1, ptl);