Re: [PATCH] blk-mq: check kobject state_in_sysfs before deleting in blk_mq_unregister_hctx

[Jens Axboe <axboe@kernel.dk>]

On 8/28/25 7:09 PM, Yu Kuai wrote:
> Hi,
> 
> ? 2025/08/29 1:23, Jens Axboe ??:
>> On 8/28/25 3:28 AM, Li Nan wrote:
>>>
>>>
>>> ? 2025/8/27 16:10, Ming Lei ??:
>>>> On Wed, Aug 27, 2025 at 11:22:06AM +0800, Li Nan wrote:
>>>>>
>>>>>
>>>>> ? 2025/8/27 9:35, Ming Lei ??:
>>>>>> On Wed, Aug 27, 2025 at 09:04:45AM +0800, Yu Kuai wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> ? 2025/08/27 8:58, Ming Lei ??:
>>>>>>>> On Tue, Aug 26, 2025 at 04:48:54PM +0800, linan666@huaweicloud.com wrote:
>>>>>>>>> From: Li Nan <linan122@huawei.com>
>>>>>>>>>
>>>>>>>>> In __blk_mq_update_nr_hw_queues() the return value of
>>>>>>>>> blk_mq_sysfs_register_hctxs() is not checked. If sysfs creation for hctx
>>>>>>>>
>>>>>>>> Looks we should check its return value and handle the failure in both
>>>>>>>> the call site and blk_mq_sysfs_register_hctxs().
>>>>>>>
>>>>>>>    From __blk_mq_update_nr_hw_queues(), the old hctxs is already
>>>>>>> unregistered, and this function is void, we failed to register new hctxs
>>>>>>> because of memory allocation failure. I really don't know how to handle
>>>>>>> the failure here, do you have any suggestions?
>>>>>>
>>>>>> It is out of memory, I think it is fine to do whatever to leave queue state
>>>>>> intact instead of making it `partial workable`, such as:
>>>>>>
>>>>>> - try update nr_hw_queues to 1
>>>>>>
>>>>>> - if it still fails, delete disk & mark queue as dead if disk is attached
>>>>>>
>>>>>
>>>>> If we ignore these non-critical sysfs creation failures, the disk remains
>>>>> usable with no loss of functionality. Deleting the disk seems to escalate
>>>>> the error?
>>>>
>>>> It is more like a workaround by ignoring the sysfs register failure. And if
>>>> the issue need to be fixed in this way, you have to document it. >
>>>> In case of OOM, it usually means that the system isn't usable any more.
>>>> But it is NOIO allocation and the typical use case is for error recovery in
>>>> nvme pci, so there may not be enough pages for noio allocation only. That is
>>>> the reason for ignoring sysfs register in blk_mq_update_nr_hw_queues()?
>>>>
>>>> But NVMe has been pretty fragile in this area by using non-owner queue
>>>> freeze, and call blk_mq_update_nr_hw_queues() on frozen queue, so it is
>>>> really necessary to take it into account?
>>>
>>> I agree with your points about NOIO and NVMe.
>>>
>>> I hit this issue in null_blk during fuzz testing with memory-fault
>>> injection. Changing the number of hardware queues under OOM is
>>> extremely rare in real-world usage. So I think adding a workaround and
>>> documenting it is sufficient. What do you think?
>>
>> Working around it is fine, as it isn't a situation we really need to
>> worry about. But let's please not do it by poking at kobject internals.
>>
> 
> There is already used in someplaces like sysfs_slab_unlink().
> 
> Do we prefre add a new hctx->state like BLK_MQ_S_REGISTERED?

If it's already used in a few spots, then I guess we should just be
using it as well rather than have a state around it. So I guess it's
fine. I'll just grab the patch.

-- 
Jens Axboe