On 7/18/25 7:31 AM, Ming Lei wrote:
> On Fri, Jul 18, 2025 at 12:13:25AM +0530, Nilay Shroff wrote:
>>
>>
>> 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.
>
> Yeah, that is another way.
>
So I will update this way in the next patch.
>>
>>>> -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?
>
> You still can add blk_mq_elv_switch_none() and blk_mq_elv_switch_back(),
> meantime call elv_update_nr_hw_queues() from blk_mq_elv_switch_back() by
> passing elevator type name.
>
> If one bug fix is simpler, it may become easier to review & merge in -rc or
> backport.
>
Okay will do this way in the next version.
Thanks,
--Nilay
