Re: [PATCH BUGFIX] block: make elevator_get robust against cross blk/blk-mq choice

[Jens Axboe]

On 02/13/2017 03:09 PM, Omar Sandoval wrote:
> On Mon, Feb 13, 2017 at 10:01:07PM +0100, Paolo Valente wrote:
>> If, at boot, a legacy I/O scheduler is chosen for a device using blk-mq,
>> or, viceversa, a blk-mq scheduler is chosen for a device using blk, then
>> that scheduler is set and initialized without any check, driving the
>> system into an inconsistent state. This commit addresses this issue by
>> letting elevator_get fail for these wrong cross choices.
>>
>> Signed-off-by: Paolo Valente <paolo.valente@xxxxxxxxxx>
>> ---
>>  block/elevator.c | 26 ++++++++++++++++++--------
>>  1 file changed, 18 insertions(+), 8 deletions(-)
> 
> Hey, Paolo,
> 
> How exactly are you triggering this? In __elevator_change(), we do check
> for mq or not mq:
> 
> 	if (!e->uses_mq && q->mq_ops) {
> 		elevator_put(e);
> 		return -EINVAL;
> 	}
> 	if (e->uses_mq && !q->mq_ops) {
> 		elevator_put(e);
> 		return -EINVAL;
> 	}
> 
> We don't ever appear to call elevator_init() with a specific scheduler
> name, and for the default we switch off of q->mq_ops and use the
> defaults from Kconfig:
> 
> 	if (q->mq_ops && q->nr_hw_queues == 1)
> 		e = elevator_get(CONFIG_DEFAULT_SQ_IOSCHED, false);
> 	else if (q->mq_ops)
> 		e = elevator_get(CONFIG_DEFAULT_MQ_IOSCHED, false);
> 	else
> 		e = elevator_get(CONFIG_DEFAULT_IOSCHED, false);
> 
> 	if (!e) {
> 		printk(KERN_ERR
> 			"Default I/O scheduler not found. " \
> 			"Using noop/none.\n");
> 		e = elevator_get("noop", false);
> 	}
> 
> So I guess this could happen if someone manually changed those Kconfig
> options, but I don't see what other case would make this happen, could
> you please explain?

Was wondering the same - is it using the 'elevator=' boot parameter?
Didn't look at that path just now, but that's the only one I could
think of. If it is, I'd much prefer only using 'chosen_elevator' for
the non-mq stuff, and the fix should be just that instead.

So instead of:

	if (!e && *chosen_elevator) {

do

	if (!e && !q->mq_ops && && *chosen_elevator) {

-- 
Jens Axboe