powerpc/mm: Show if a bad page fault on data is read or write.

DSISR (or ESR on some CPUs) has a bit to tell if the fault is due to a
read or a write.

Display it.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Santosh Sivaraj <santosh@fossix.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/4f88d7e6fda53b5f80a71040ab400242f6c8cb93.1566400889.git.christophe.leroy@c-s.fr

arch/powerpc/mm/fault.c

@@ -645,6 +645,7 @@ NOKPROBE_SYMBOL(do_page_fault);
 void bad_page_fault(struct pt_regs *regs, unsigned long address, int sig)
 {
 	const struct exception_table_entry *entry;
+	int is_write = page_fault_is_write(regs->dsisr);
 
 	/* Are we prepared to handle this fault?  */
 	if ((entry = search_exception_tables(regs->nip)) != NULL) {
@@ -658,9 +659,10 @@ void bad_page_fault(struct pt_regs *regs, unsigned long address, int sig)
 	case 0x300:
 	case 0x380:
 	case 0xe00:
-		pr_alert("BUG: %s at 0x%08lx\n",
+		pr_alert("BUG: %s on %s at 0x%08lx\n",
 			 regs->dar < PAGE_SIZE ? "Kernel NULL pointer dereference" :
-			 "Unable to handle kernel data access", regs->dar);
+			 "Unable to handle kernel data access",
+			 is_write ? "write" : "read", regs->dar);
 		break;
 	case 0x400:
 	case 0x480: