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From: Hou Tao <>
To: Ming Lei <>
Cc: Jens Axboe <>, <>
Subject: Re: [PATCH] block: check the existence of tags[hctx_idx] in blk_mq_clear_rq_mapping()
Date: Thu, 9 Sep 2021 19:14:20 +0800	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <YTnRsxec3JfPINmu@T590>


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].



> Thanks,
> Ming
> .

  reply	other threads:[~2021-09-09 11:14 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-09  9:00 [PATCH] block: check the existence of tags[hctx_idx] in blk_mq_clear_rq_mapping() Hou Tao
2021-09-09  9:19 ` Ming Lei
2021-09-09 11:14   ` Hou Tao [this message]
2021-09-13  1:43     ` Ming Lei

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