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

From: Omar Sandoval
Date: Mon Feb 13 2017 - 17:09:42 EST

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) {
return -EINVAL;
if (e->uses_mq && !q->mq_ops) {
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);
e = elevator_get(CONFIG_DEFAULT_IOSCHED, false);

if (!e) {
"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?