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

[Ming Lei <ming.lei@redhat.com>]

On Thu, Aug 28, 2025 at 05:28:26PM +0800, Li Nan wrote:
> 
> 
> 在 2025/8/27 16:10, Ming Lei 写道:
> > On Wed, Aug 27, 2025 at 11:22:06AM +0800, Li Nan wrote:
> > > 
> > > 
> > > 在 2025/8/27 9:35, Ming Lei 写道:
> > > > On Wed, Aug 27, 2025 at 09:04:45AM +0800, Yu Kuai wrote:
> > > > > Hi,
> > > > > 
> > > > > 在 2025/08/27 8:58, Ming Lei 写道:
> > > > > > 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?

Looks fine for me.


Thanks, 
Ming