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

[Jens Axboe]

On 02/13/2017 03:28 PM, Jens Axboe wrote:
> 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) {

Confirmed, that's what it seems to be, and here's a real diff of the
above example that works for me:

diff --git a/block/elevator.c b/block/elevator.c
index 27ff1ed5a6fa..699d10f71a2c 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -207,11 +207,12 @@ int elevator_init(struct request_queue *q, char *name)
 	}
 
 	/*
-	 * Use the default elevator specified by config boot param or
-	 * config option.  Don't try to load modules as we could be running
-	 * off async and request_module() isn't allowed from async.
+	 * Use the default elevator specified by config boot param for
+	 * non-mq devices, or by config option. Don't try to load modules
+	 * as we could be running off async and request_module() isn't
+	 * allowed from async.
 	 */
-	if (!e && *chosen_elevator) {
+	if (!e && !q->mq_ops && *chosen_elevator) {
 		e = elevator_get(chosen_elevator, false);
 		if (!e)
 			printk(KERN_ERR "I/O scheduler %s not found\n",

-- 
Jens Axboe