linux-kernel - blk-mq: should elv_iosched

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Message-ID: <20141231003726.00002588@localhost>
Date:	Wed, 31 Dec 2014 00:37:26 +0100
From:	Sebastian Herbszt <herbszt@....de>
To:	linux-kernel@...r.kernel.org
Cc:	Jens Axboe <axboe@...nel.dk>, Sebastian Herbszt <herbszt@....de>
Subject: blk-mq: should elv_iosched_store return ENXIO/EINVAL?

Hello,

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
 failed

With blk-mq no error is returned:

# cat /sys/block/sda/queue/scheduler
none
# echo foo > /sys/block/sda/queue/scheduler
# echo $?
0


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;
 992
 993         if (!q->elevator)
 994                 return count;
 995
 996         ret = __elevator_change(q, name);

and

 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;
 956
 957         if (!q->elevator)
 958                 return -ENXIO;
 959
 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?

Sebastian
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