mm: allow preemption in apply_to_pte_range

Impact: allow preemption in apply_to_pte_range updates to init_mm

Preemption is now allowed for lazy mmu mode, so don't disable
it for the inner loop of apply_to_pte_range.  This only applies
when doing updates to init_mm; user pagetables are still modified
under the pte lock, so preemption is disabled anyway.
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

mm/memory.c

@@ -1718,7 +1718,6 @@ 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);
@@ -1730,7 +1729,6 @@ 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);