block: fix default IO priority handling

The default IO priority is the best effort (BE) class with the
normal priority level IOPRIO_NORM (4). However, get_task_ioprio()
returns IOPRIO_CLASS_NONE/IOPRIO_NORM as the default priority and
get_current_ioprio() returns IOPRIO_CLASS_NONE/0. Let's be consistent
with the defined default and have both of these functions return the
default priority IOPRIO_PRIO_VALUE(IOPRIO_CLASS_BE, IOPRIO_NORM) when
the user did not define another default IO priority for the task.

In include/uapi/linux/ioprio.h, introduce the IOPRIO_BE_NORM macro as
an alias to IOPRIO_NORM to clarify that this default level applies to
the BE priotity class. In include/linux/ioprio.h, define the macro
IOPRIO_DEFAULT as IOPRIO_PRIO_VALUE(IOPRIO_CLASS_BE, IOPRIO_BE_NORM)
and use this new macro when setting a priority to the default.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Link: https://lore.kernel.org/r/20210811033702.368488-7-damien.lemoal@wdc.com
[axboe: drop unnecessary lightnvm change]
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-iosched.c

@@ -5411,7 +5411,7 @@ static struct bfq_queue **bfq_async_queue_prio(struct bfq_data *bfqd,
 	case IOPRIO_CLASS_RT:
 		return &bfqg->async_bfqq[0][ioprio];
 	case IOPRIO_CLASS_NONE:
-		ioprio = IOPRIO_NORM;
+		ioprio = IOPRIO_BE_NORM;
 		fallthrough;
 	case IOPRIO_CLASS_BE:
 		return &bfqg->async_bfqq[1][ioprio];

block/ioprio.c

@@ -170,7 +170,7 @@ static int get_task_ioprio(struct task_struct *p)
 	ret = security_task_getioprio(p);
 	if (ret)
 		goto out;
-	ret = IOPRIO_PRIO_VALUE(IOPRIO_CLASS_NONE, IOPRIO_NORM);
+	ret = IOPRIO_DEFAULT;
 	task_lock(p);
 	if (p->io_context)
 		ret = p->io_context->ioprio;
@@ -182,9 +182,9 @@ out:
 int ioprio_best(unsigned short aprio, unsigned short bprio)
 {
 	if (!ioprio_valid(aprio))
-		aprio = IOPRIO_PRIO_VALUE(IOPRIO_CLASS_BE, IOPRIO_NORM);
+		aprio = IOPRIO_DEFAULT;
 	if (!ioprio_valid(bprio))
-		bprio = IOPRIO_PRIO_VALUE(IOPRIO_CLASS_BE, IOPRIO_NORM);
+		bprio = IOPRIO_DEFAULT;
 
 	return min(aprio, bprio);
 }

include/linux/ioprio.h

@@ -8,6 +8,11 @@
 
 #include <uapi/linux/ioprio.h>
 
+/*
+ * Default IO priority.
+ */
+#define IOPRIO_DEFAULT	IOPRIO_PRIO_VALUE(IOPRIO_CLASS_BE, IOPRIO_BE_NORM)
+
 /*
  * Check that a priority value has a valid class.
  */
@@ -51,7 +56,7 @@ static inline int get_current_ioprio(void)
 
 	if (ioc)
 		return ioc->ioprio;
-	return IOPRIO_PRIO_VALUE(IOPRIO_CLASS_NONE, 0);
+	return IOPRIO_DEFAULT;
 }
 
 /*

include/uapi/linux/ioprio.h

@@ -44,8 +44,9 @@ enum {
 };
 
 /*
- * Fallback BE priority
+ * Fallback BE priority level.
  */
-#define IOPRIO_NORM	(4)
+#define IOPRIO_NORM	4
+#define IOPRIO_BE_NORM	IOPRIO_NORM
 
 #endif /* _UAPI_LINUX_IOPRIO_H */