[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 <heiko.carstens@de.ibm.com>
Signed-off-by: Carsten Otte <cotte@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/pgtable.h

@@ -696,6 +696,7 @@ void gmap_disable(struct gmap *gmap);
 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

@@ -307,7 +307,7 @@ static inline int do_exception(struct pt_regs *regs, int access,
 
 #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

@@ -393,7 +393,10 @@ int gmap_map_segment(struct gmap *gmap, unsigned long from,
 }
 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;
@@ -461,7 +464,17 @@ unsigned long gmap_fault(unsigned long address, struct gmap *gmap)
 		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);