On Thu, Sep 09, 2021 at 07:14:20PM +0800, Hou Tao wrote: > Hi, > > On 9/9/2021 5:19 PM, Ming Lei wrote: > > Hello Hou, > > > > On Thu, Sep 09, 2021 at 05:00:54PM +0800, Hou Tao wrote: > >> According to commit 4412efecf7fd ("Revert "blk-mq: remove code for > >> dealing with remapping queue""), for some devices queue hctx may not > >> being mapped, and tagset->tags[hctx_idx] will be released and be NULL. > >> > >> If an IO scheduler is used on these devices, blk_mq_clear_rq_mapping() > >> will be called for all hctxs in blk_mq_sched_free_requests() during > >> scheduler switch, and these will be oops. So checking the existence of > >> tags[hctx_idx] before going on in blk_mq_clear_rq_mapping(). > > unmapped hctx should be caused by blk_mq_update_nr_hw_queues() only, > > but scheduler tags is updated there too, so not sure it is one real > > issue, did you observe such kernel panic? any kernel log? > Not a real issue, just find the potential "problem" during code review. > > But is the case below possible ? > There is an unmapped hctx and a freed tags[hctx_idx] after > blk_mq_update_nr_hw_queues(), and IO scheduler is used. When > switching IO scheduler to none, the previous schedule tag > on each hctx will be freed in blk_mq_sched_free_requests(). > blk_mq_sched_free_requests() will call blk_mq_free_rqs(), and > > blk_mq_free_rqs() will access the NULLed tags[hctx_idx]. It isn't possible. One invariant is that the first 'q->nr_hw_queues' elements of ->queue_hw_ctx[] / ->tags[] are valid and the others are freed after blk_mq_update_nr_hw_queues() returns. thanks, Ming