linux-kernel - Re: [PATCH] blk-mq: Properly init bios from blk_mq_alloc_request

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Date:   Sun, 23 Oct 2022 21:12:44 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     John Garry <john.garry@...wei.com>
Cc:     axboe@...nel.dk, linux-kernel@...r.kernel.org,
        linux-block@...r.kernel.org, hch@....de
Subject: Re: [PATCH] blk-mq: Properly init bios from
 blk_mq_alloc_request_hctx()

On Sun, Oct 23, 2022 at 12:07:26AM +0800, John Garry wrote:
> Function blk_mq_alloc_request_hctx() is missing zeroing/init of rq->bio,
> biotail, __sector, and __data_len members, which blk_mq_alloc_request()
> has.
> 
> Move init'ing of those members to common blk_mq_rq_ctx_init().
> 
> Fixes: 1f5bd336b9150 ("blk-mq: add blk_mq_alloc_request_hctx")
> Suggested-by: Bart Van Assche <bvanassche@....org>
> Signed-off-by: John Garry <john.garry@...wei.com>
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 8070b6c10e8d..260adeb2e455 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -402,6 +402,10 @@ static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
>  		}
>  	}
>  
> +	rq->__data_len = 0;
> +	rq->__sector = (sector_t) -1;
> +	rq->bio = rq->biotail = NULL;
> +
>  	return rq;
>  }
>  
> @@ -591,9 +595,6 @@ struct request *blk_mq_alloc_request(struct request_queue *q, blk_opf_t opf,
>  		if (!rq)
>  			goto out_queue_exit;
>  	}
> -	rq->__data_len = 0;
> -	rq->__sector = (sector_t) -1;
> -	rq->bio = rq->biotail = NULL;

This patch looks not good, why do you switch to initialize the three fields
twice in fast path?

BTW, we know blk_mq_alloc_request_hctx() has big trouble, so please
avoid to extend it to other use cases.

Thanks,
Ming