Re: [PATCHv2] block: restore two stage elevator switch while running nr_hw_queue update

Yu Kuai
2025-07-25T01:15:40Z

Hi,

在 2025/07/23 19:05, Nilay Shroff 写道:
>
>
> On 7/23/25 12:28 PM, Yu Kuai wrote:
>>>>>> BTW, this is not related to this patch. Should we handle fall_back
>>>>>> failure like blk_mq_sysfs_register_hctxs()?
>>>>>>
>>>>> OKay I guess you meant here handle failure case by unwinding the
>>>>> queue instead of looping through it from start to end. If yes, then
>>>>> it could be done but again we may not want to do it the bug fix patch.
>>>>>
>>>>
>>>> Not like that, actually I don't have any ideas for now, the hctxs is
>>>> unregistered first, and if register failed, for example, due to -ENOMEM,
>>>> I can't find a way to fallback :(
>>>>
>>> If registering new hctxs fails, we fall back to the previous value of
>>> nr_hw_queues (prev_nr_hw_queues). When prev_nr_hw_queues is less than
>>> the new nr_hw_queues, we do not reallocate memory for the existing hctxs—
>>> instead, we reuse the memory that was already allocated.
>>>
>>> Memory allocation is only attempted for the additional hctxs beyond
>>> prev_nr_hw_queues. Therefore, if memory allocation for these new hctxs
>>> fails, we can safely fall back to prev_nr_hw_queues because the memory
>>> of the previously allocated hctxs remains intact.
>>
>> No, like I said before, blk_mq_sysfs_unregister_hctxs() will free memory
>> by kobject_del() for hctx->kobj and ctx->kobj, and
>> __blk_mq_update_nr_hw_queues() call that helper in the beginning.
>> And later in the fall back code, blk_mq_sysfs_register_hctxs() can fail
>> by memory allocation in kobject_add(), however, the return value is not
>> checked.
>>
> This can be done checking the kobject state in sysfs: kobj->state_in_sysfs.
> If kobj->state_in_sysfs is 1 then it implies that kobject_add() for this
> kobj was successful and we can safely call kobject_del() on it otherwise
> we can skip it. We already have few places in the kernel using this trick.
> For instance, check sysfs_slab_unlink(). So, IMO, similar technique could be
> used for hctx->kobj and ctx->kobj as well while we attempt to delete these
> kobjects from unregistering queue and nr_hw_queue update.

I don't like missing sysfs attributes, however I don't have better
solution, I'm good with this.

Thanks,
Kuai

>
> Thanks,
> --Nilay
> .
>