On Thu, Aug 14, 2025 at 11:35:14AM +0800, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> ---
> 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@xxxxxxxxxx>
Thanks,
Ming
