On 7/17/25 8:14 PM, Ming Lei wrote:
>> +static int blk_mq_elv_switch_none(struct request_queue *q,
>> + struct xarray *elv_tbl)
>> +{
>> + int ret = 0;
>> +
>> + lockdep_assert_held_write(&q->tag_set->update_nr_hwq_lock);
>> +
>> + /*
>> + * Accessing q->elevator without holding q->elevator_lock is safe here
>> + * because we're called from nr_hw_queue update which is protected by
>> + * set->update_nr_hwq_lock in the writer context. So, scheduler update/
>> + * switch code (which acquires the same lock in the reader context)
>> + * can't run concurrently.
>> + */
>> + if (q->elevator) {
>> + char *elevator_name = (char *)q->elevator->type->elevator_name;
>> +
>> + ret = xa_insert(elv_tbl, q->id, elevator_name, GFP_KERNEL);
>
> This way isn't memory safe, because elevator module can be reloaded
> during updating nr_hw_queues. One solution is to build a string<->int mapping
> table, and store the (int) index of elevator type to xarray.
>
Yes good point! How about avoiding this issue by using __elevator_get() and
elevator_put()? We can take module reference while switching elevator to 'none'
and then put the module reference while we switch back elevator.
>> -void elv_update_nr_hw_queues(struct request_queue *q)
>> -{
>> - struct elv_change_ctx ctx = {};
>> - int ret = -ENODEV;
>> -
>> - WARN_ON_ONCE(q->mq_freeze_depth == 0);
>> -
>> - mutex_lock(&q->elevator_lock);
>> - if (q->elevator && !blk_queue_dying(q) && blk_queue_registered(q)) {
>> - ctx.name = q->elevator->type->elevator_name;
>> -
>> - /* force to reattach elevator after nr_hw_queue is updated */
>> - ret = elevator_switch(q, &ctx);
>> - }
>> - mutex_unlock(&q->elevator_lock);
>> - blk_mq_unfreeze_queue_nomemrestore(q);
>> - if (!ret)
>> - WARN_ON_ONCE(elevator_change_done(q, &ctx));
>> -}
>> -
>> /*
>> * Use the default elevator settings. If the chosen elevator initialization
>> * fails, fall back to the "none" elevator (no elevator).
>> diff --git a/block/elevator.h b/block/elevator.h
>> index a07ce773a38f..440b6e766848 100644
>> --- a/block/elevator.h
>> +++ b/block/elevator.h
>> @@ -85,6 +85,17 @@ struct elevator_type
>> struct list_head list;
>> };
>>
>> +/* Holding context data for changing elevator */
>> +struct elv_change_ctx {
>> + const char *name;
>> + bool no_uevent;
>> +
>> + /* for unregistering old elevator */
>> + struct elevator_queue *old;
>> + /* for registering new elevator */
>> + struct elevator_queue *new;
>> +};
>> +
>
> You may avoid the big chunk of code move for both `elv_change_ctx` and
> `elv_update_nr_hw_queues()`, not sure why you did that in a bug fix patch.
>
This is because I’ve reintroduced the blk_mq_elv_switch_none and
blk_mq_elv_switch_back functions. I replaced elv_update_nr_hw_queues with
blk_mq_elv_switch_back(), which was the approach used prior to commit
596dce110b7d ("block: simplify elevator reattachment for updating nr_hw_queues").
Both blk_mq_elv_switch_none and blk_mq_elv_switch_back are defined in blk-mq.c
and use the elevator APIs for switching elevators. These APIs — namely
elevator_switch and elevator_change_done — rely on the elv_change_ctx structure.
As a result, I had to move some code around to ensure elv_change_ctx is accessible
from within blk-mq.c.
While it would be possible to avoid this code movement by continuing to use
elv_update_nr_hw_queues, I felt it was cleaner to restore the two-stage elevator
switch more fully by reintroducing the blk_mq_elv_switch_none and blk_mq_elv_switch_back
APIs, which were used prior to the simplification in the aforementioned commit.
Do you see any concerns with this code movement? Or is such restructuring generally
avoided in a bug fix patch?
Thanks,
--Nilay
