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Date:   Wed, 15 Jun 2022 08:04:42 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Jens Axboe <axboe@kernel.dk>, Christoph Hellwig <hch@lst.de>,
        linux-block@vger.kernel.org, Jan Kara <jack@suse.cz>
Subject: Re: [PATCH 2/3] blk-mq: avoid to touch q->elevator without any
 protection
Message-ID: <20220615060442.GD22115@lst.de>
References: <20220615023712.750122-1-ming.lei@redhat.com> <20220615023712.750122-3-ming.lei@redhat.com>
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> +++ b/block/elevator.c
> @@ -612,6 +612,16 @@ int elevator_switch_mq(struct request_queue *q,
>  		}
>  	}
>  
> +	/*
> +	 * Is the request queue handled by an IO scheduler that does not
> +	 * respect hardware queues when dispatching?
> +	 */
> +	if (new_e && new_e->ops.dispatch_request &&
> +	    !(new_e->elevator_features & ELEVATOR_F_MQ_AWARE))
> +		blk_queue_flag_set(QUEUE_FLAG_SQ_SCHED, q);
> +	else
> +		blk_queue_flag_clear(QUEUE_FLAG_SQ_SCHED, q);

Please just set the QUEUE_FLAG_SQ_SCHED flag directly from the
mq-deadline and bfq scheduler ans drop the ELEVATOR_F_MQ_AWARE
flag.

Otherwise this approach looks good.