s390/pageattr: do a single TLB flush for change_page_attr

The change of the access rights for an address range in the kernel
address space is currently done with a loop of IPTE + a store of the
modified PTE. Between the IPTE and the store the PTE will be invalid,
this intermediate state can cause problems with concurrent accesses.

Consider a change of a kernel area from read-write to read-only, a
concurrent reader of that area should be fine but with the invalid
PTE it might get an unexpected exception.

Remove the IPTEs for each PTE and do a global flush after all PTEs
have been modified.
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/mm/pageattr.c

@@ -65,19 +65,17 @@ static pte_t *walk_page_table(unsigned long addr)
 static void change_page_attr(unsigned long addr, int numpages,
 			     pte_t (*set) (pte_t))
 {
-	pte_t *ptep, pte;
+	pte_t *ptep;
 	int i;
 
 	for (i = 0; i < numpages; i++) {
 		ptep = walk_page_table(addr);
 		if (WARN_ON_ONCE(!ptep))
 			break;
-		pte = *ptep;
-		pte = set(pte);
-		__ptep_ipte(addr, ptep);
-		*ptep = pte;
+		*ptep = set(*ptep);
 		addr += PAGE_SIZE;
 	}
+	__tlb_flush_kernel();
 }
 
 int set_memory_ro(unsigned long addr, int numpages)