Re: [PATCH 08/16] blk-mq: fix blk_mq_tags double free while nr_requests grown

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

On Thu, Aug 14, 2025 at 11:35:14AM +0800, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@huawei.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 halper blk_mq_alloc_sched_tags() to
> allocate a new sched_tags. 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@huawei.com>
> ---
>  block/blk-mq.c | 31 ++++++++++++++++++++-----------
>  1 file changed, 20 insertions(+), 11 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index a7d6a20c1524..f1c11f591c27 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -4917,6 +4917,23 @@ void blk_mq_free_tag_set(struct blk_mq_tag_set *set)
>  }
>  EXPORT_SYMBOL(blk_mq_free_tag_set);
>  
> +static int blk_mq_sched_grow_tags(struct request_queue *q, unsigned int nr)
> +{
> +	struct elevator_tags *et =
> +		blk_mq_alloc_sched_tags(q->tag_set, q->nr_hw_queues, nr);
> +	struct blk_mq_hw_ctx *hctx;
> +	unsigned long i;
> +
> +	if (!et)
> +		return -ENOMEM;
> +
> +	blk_mq_free_sched_tags(q->elevator->et, q->tag_set);
> +	queue_for_each_hw_ctx(q, hctx, i)
> +		hctx->sched_tags = et->tags[i];
> +	q->elevator->et = et;
> +	return 0;
> +}

It depends on protection from elevator_lock, so probably it is
helpful by adding lockdep_assert_held(&q->elevator_lock), otherwise
this fix looks fine:

Reviewed-by: Ming Lei <ming.lei@redhat.com>


Thanks,
Ming