linux-kernel - Re: [PATCH 08/10] blk-mq: fix blk_mq_tags double free while nr

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Message-ID: <af40ef99-9b61-4725-ba77-c5d3741add99@kernel.org>
Date: Sat, 16 Aug 2025 10:57:23 +0800
From: Yu Kuai <yukuai@...nel.org>
To: Nilay Shroff <nilay@...ux.ibm.com>, Yu Kuai <yukuai1@...weicloud.com>,
 axboe@...nel.dk, hare@...e.de, ming.lei@...hat.com
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
 yukuai3@...wei.com, yi.zhang@...wei.com, yangerkun@...wei.com,
 johnny.chenyi@...wei.com
Subject: Re: [PATCH 08/10] blk-mq: fix blk_mq_tags double free while
 nr_requests grown

Hi,

在 2025/8/16 3:30, Nilay Shroff 写道:
>
> On 8/15/25 1:32 PM, Yu Kuai wrote:
>> From: Yu Kuai <yukuai3@...wei.com>
>>
>> In the case user trigger tags grow by queue sysfs attribute nr_requests,
>> hctx->sched_tags will be freed directly and replaced with a new
>> allocated tags, see blk_mq_tag_update_depth().
>>
>> The problem is that hctx->sched_tags is from elevator->et->tags, while
>> et->tags is still the freed tags, hence later elevator exist will try to
>> free the tags again, causing kernel panic.
>>
>> Fix this problem by using new allocated elevator_tags, also convert
>> blk_mq_update_nr_requests to void since this helper will never fail now.
>>
>> Meanwhile, there is a longterm problem can be fixed as well:
>>
>> If blk_mq_tag_update_depth() succeed for previous hctx, then bitmap depth
>> is updated, however, if following hctx failed, q->nr_requests is not
>> updated and the previous hctx->sched_tags endup bigger than q->nr_requests.
>>
>> Fixes: f5a6604f7a44 ("block: fix lockdep warning caused by lock dependency in elv_iosched_store")
>> Fixes: e3a2b3f931f5 ("blk-mq: allow changing of queue depth through sysfs")
>> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
>> ---
>>   block/blk-mq.c    | 19 ++++++-------------
>>   block/blk-mq.h    |  4 +++-
>>   block/blk-sysfs.c | 21 ++++++++++++++-------
>>   3 files changed, 23 insertions(+), 21 deletions(-)
>>
>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>> index 11c8baebb9a0..e9f037a25fe3 100644
>> --- a/block/blk-mq.c
>> +++ b/block/blk-mq.c
>> @@ -4917,12 +4917,12 @@ void blk_mq_free_tag_set(struct blk_mq_tag_set *set)
>>   }
>>   EXPORT_SYMBOL(blk_mq_free_tag_set);
>>   
>> -int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr)
>> +void blk_mq_update_nr_requests(struct request_queue *q,
>> +			       struct elevator_tags *et, unsigned int nr)
>>   {
>>   	struct blk_mq_tag_set *set = q->tag_set;
>>   	struct blk_mq_hw_ctx *hctx;
>>   	unsigned long i;
>> -	int ret = 0;
>>   
>>   	blk_mq_quiesce_queue(q);
>>   
>> @@ -4946,24 +4946,17 @@ int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr)
>>   				nr - hctx->sched_tags->nr_reserved_tags);
>>   		}
>>   	} else {
>> -		queue_for_each_hw_ctx(q, hctx, i) {
>> -			if (!hctx->tags)
>> -				continue;
>> -			ret = blk_mq_tag_update_depth(hctx, &hctx->sched_tags,
>> -						      nr);
>> -			if (ret)
>> -				goto out;
>> -		}
>> +		blk_mq_free_sched_tags(q->elevator->et, set);
> I think you also need to ensure that elevator tags are freed after we unfreeze
> queue and release ->elevator_lock otherwise we may get into the lockdep splat
> for pcpu_lock dependency on ->freeze_lock and/or ->elevator_lock. Please note
> that blk_mq_free_sched_tags internally invokes sbitmap_free which invokes
> free_percpu which acquires pcpu_lock.

Ok, thanks for the notice. However, as Ming suggested, we might fix this 
problem

in the next merge window. I'll send one patch to fix this regression by 
replace

st->tags with reallocated new sched_tags as well.


Thanks,

Kuai

>   
> Thanks,
> --Nilay
>
>
>