Re: blk-mq: should elv_iosched_store return ENXIO/EINVAL?

From: Jens Axboe
Date: Tue Dec 30 2014 - 20:49:53 EST

On 12/30/2014 04:37 PM, Sebastian Herbszt wrote:

setting an invalid elevator without blk-mq results in an error:

# cat /sys/block/sda/queue/scheduler
noop deadline [cfq]
# echo foo > /sys/block/sda/queue/scheduler
-bash: echo: write error: Invalid argument
# dmesg
[ 328.767088] elevator: type foo not found
[ 328.767097] elevator: switch to foo

With blk-mq no error is returned:

# cat /sys/block/sda/queue/scheduler
# echo foo > /sys/block/sda/queue/scheduler
# echo $?

block/elevator.c got

988 ssize_t elv_iosched_store(struct request_queue *q, const char *name,
989 size_t count)
990 {
991 int ret;
993 if (!q->elevator)
994 return count;
996 ret = __elevator_change(q, name);


952 static int __elevator_change(struct request_queue *q, const char *name)
953 {
954 char elevator_name[ELV_NAME_MAX];
955 struct elevator_type *e;
957 if (!q->elevator)
958 return -ENXIO;
960 strlcpy(elevator_name, name, sizeof(elevator_name));
961 e = elevator_get(strstrip(elevator_name), true);
962 if (!e) {
963 printk(KERN_ERR "elevator: type %s not found\n", elevator_name);
964 return -EINVAL;
965 }

So !q->elevator is checked in elv_iosched_store and __elevator_change.

Should elv_iosched_store return ENXIO or EINVAL or should __elevator_change
handle this?

I agree the behavior is strange, but it actually matches what would happen for a make_request_fn based driver in this or earlier kernels. So there is a worry of changing the API if we modify it in general. The safe change would be to have these two lines before the q->elevator check:

if (q->mq_ops)
return -EINVAL;

since that's new enough not to be a "real" API change. If we do that, we could let it slide into the general !q->elevator case after a few revisions.

Or we can just leave it as-is. If you read back the value after writing to it, it will always return "none".

Jens Axboe

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