Re: [BUG report] WARNING of sysfs in __blk_mq_update_nr_hw_queues()

On 2/28/25 7:52 AM, Li Nan wrote:

Hi,

In __blk_mq_update_nr_hw_queues(), we don't check the return value of
blk_mq_sysfs_register_hctxs(). When sysfs creation fails, there's no
proper error handling. This leads to a kernel warning during subsequent
__blk_mq_update_nr_hw_queues() calls or disk removal:

kernfs: can not remove 'nr_tags', no directory
WARNING: CPU: 2 PID: 805 at fs/kernfs/dir.c:1703 kernfs_remove_by_name_ns+0x12e/0x140
Call Trace:
 <TASK>
 remove_files+0x39/0xb0
 sysfs_remove_group+0x48/0xf0
 sysfs_remove_groups+0x31/0x60
 __kobject_del+0x23/0xf0
 kobject_del+0x17/0x40
 blk_mq_unregister_hctx+0x5d/0x80
 blk_mq_sysfs_unregister_hctxs+0x89/0xd0
 blk_mq_update_nr_hw_queues+0x31c/0x820
 nullb_update_nr_hw_queues+0x71/0xe0 [null_blk]
 nullb_device_submit_queues_store+0xa4/0x130 [null_blk]

Should we add error checking for blk_mq_sysfs_register_hctxs() and
propagate the error to abort the update operation when it fails? This
would prevent subsequent operations from hitting invalid sysfs entries.

IMO, yes error checking should be added here. However it will be tricky
to undo everything as the error might have happened deep inside loop. We
need to carefully delete all sysfs objects added under each hctx->kobj.
BTW, typically, we don't abort the nr_hw_queue update operation but 
instead fallback to the previously configured number of hw queues.

Thanks,
--Nilay