[BLOCK] elevator: default choice selection

My previous default iosched patch did a poor job dealing with the
'elevator=' boot-time option.  The old behavior falls back to the
compiled-in default if the requested one is not registered at boot
time.  This patch dynamically evaluates which default
to use, and emits a suitable error message when the requested scheduler
is not available.  It also does the 'as' -> 'anticipatory' conversion
before elevator registration, which along with a modified registration
function, allows it to correctly indicate which default scheduler is
in use.

Tested for a range of boot options on 2.6.16-rc1-mm2.
Signed-off-by: Nate Diller <nate.diller@gmail.com>
Signed-off-by: Jens Axboe <axboe@suse.de>

block/elevator.c

@@ -139,34 +139,15 @@ static int elevator_attach(request_queue_t *q, struct elevator_type *e,
 
 static char chosen_elevator[16];
 
-static void elevator_setup_default(void)
+static int __init elevator_setup(char *str)
 {
-	struct elevator_type *e;
-
-	/*
-	 * If default has not been set, use the compiled-in selection.
-	 */
-	if (!chosen_elevator[0])
-		strcpy(chosen_elevator, CONFIG_DEFAULT_IOSCHED);
-
 	/*
 	 * Be backwards-compatible with previous kernels, so users
 	 * won't get the wrong elevator.
 	 */
-	if (!strcmp(chosen_elevator, "as"))
+	if (!strcmp(str, "as"))
 		strcpy(chosen_elevator, "anticipatory");
-
- 	/*
- 	 * If the given scheduler is not available, fall back to the default
- 	 */
- 	if ((e = elevator_find(chosen_elevator)))
-		elevator_put(e);
 	else
- 		strcpy(chosen_elevator, CONFIG_DEFAULT_IOSCHED);
-}
-
-static int __init elevator_setup(char *str)
-{
 		strncpy(chosen_elevator, str, sizeof(chosen_elevator) - 1);
 	return 0;
 }
@@ -184,15 +165,15 @@ int elevator_init(request_queue_t *q, char *name)
 	q->end_sector = 0;
 	q->boundary_rq = NULL;
 
-	elevator_setup_default();
-
-	if (!name)
-		name = chosen_elevator;
-
-	e = elevator_get(name);
-	if (!e)
+	if (name && !(e = elevator_get(name)))
 		return -EINVAL;
 
+	if (!e && !(e = elevator_get(chosen_elevator))) {
+		e = elevator_get(CONFIG_DEFAULT_IOSCHED);
+		if (*chosen_elevator)
+			printk("I/O scheduler %s not found\n", chosen_elevator);
+	}
+
 	eq = kmalloc(sizeof(struct elevator_queue), GFP_KERNEL);
 	if (!eq) {
 		elevator_put(e);
@@ -669,7 +650,9 @@ int elv_register(struct elevator_type *e)
 	spin_unlock_irq(&elv_list_lock);
 
 	printk(KERN_INFO "io scheduler %s registered", e->elevator_name);
-	if (!strcmp(e->elevator_name, chosen_elevator))
+	if (!strcmp(e->elevator_name, chosen_elevator) ||
+			(!*chosen_elevator &&
+			 !strcmp(e->elevator_name, CONFIG_DEFAULT_IOSCHED)))
 				printk(" (default)");
 	printk("\n");
 	return 0;