[S390] take mmap_sem when walking guest page table

gmap_fault needs to walk the guest page table. However, parts of
that may change if some other thread does munmap. In that case
gmap_unmap_notifier will also unmap the corresponding parts from
the guest page table. We need to take mmap_sem in order to serialize
these operations.
do_exception now calls __gmap_fault with mmap_sem held which does
not get exported to modules. The exported function, which is called
from KVM, now takes mmap_sem.

Reported-by: Heiko Carstens <[e-mail]>
Signed-off-by: Carsten Otte <[e-mail]>
Signed-off-by: Martin Schwidefsky <[e-mail]>

arch/s390/include/asm/pgtable.h

···
 int gmap_map_segment(struct gmap *gmap, unsigned long from,
 		     unsigned long to, unsigned long length);
 int gmap_unmap_segment(struct gmap *gmap, unsigned long to, unsigned long len);
+unsigned long __gmap_fault(unsigned long address, struct gmap *);
 unsigned long gmap_fault(unsigned long address, struct gmap *);
 
 /*

arch/s390/mm/fault.c

···
 
 #ifdef CONFIG_PGSTE
 	if (test_tsk_thread_flag(current, TIF_SIE) && S390_lowcore.gmap) {
-		address = gmap_fault(address,
+		address = __gmap_fault(address,
 				     (struct gmap *) S390_lowcore.gmap);
 		if (address == -EFAULT) {
 			fault = VM_FAULT_BADMAP;

arch/s390/mm/pgtable.c

···
 }
 EXPORT_SYMBOL_GPL(gmap_map_segment);
 
-unsigned long gmap_fault(unsigned long address, struct gmap *gmap)
+/*
+ * this function is assumed to be called with mmap_sem held
+ */
+unsigned long __gmap_fault(unsigned long address, struct gmap *gmap)
 {
 	unsigned long *table, vmaddr, segment;
 	struct mm_struct *mm;
···
 		return vmaddr | (address & ~PMD_MASK);
 	}
 	return -EFAULT;
+}
 
+unsigned long gmap_fault(unsigned long address, struct gmap *gmap)
+{
+	unsigned long rc;
+
+	down_read(&gmap->mm->mmap_sem);
+	rc = __gmap_fault(address, gmap);
+	up_read(&gmap->mm->mmap_sem);
+
+	return rc;
 }
 EXPORT_SYMBOL_GPL(gmap_fault);