[dm-devel] [PATCH] dm rq: clear cloned bio ->bi_bdev to fix I/O accounting

[Benjamin Marzinski]

On Sun, Jan 16, 2022 at 11:51:17PM -0800, Christoph Hellwig wrote:
> So I actually noticed this during code inspection a while ago, but I
> think we need to use the actual underlying device instead of NULL here
> to keep our block layer gurantees.  See the patch in my queue below.

Makes sense.

> ---
> >From 1e330b8e57fc0d6c6fb07c0ec2915dca5d7a585a Mon Sep 17 00:00:00 2001
> From: Christoph Hellwig <hch at lst.de>
> Date: Thu, 13 Jan 2022 10:53:59 +0100
> Subject: block: assign bi_bdev for cloned bios in blk_rq_prep_clone
> 
> The cloned bios for the cloned request in blk_rq_prep_clone currently
> still point to the original bi_bdev.  This is harmless because dm-mpath
> doesn't look at bi_bdev but violates the invariants of always having
> the correct bi_bdev.
> 
> Signed-off-by: Christoph Hellwig <hch at lst.de>
> ---
>  block/blk-mq.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index a6d4780580fcd..b5e35e63adad4 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2976,6 +2976,7 @@ int blk_rq_prep_clone(struct request *rq, struct request *rq_src,
>  		bio = bio_clone_fast(bio_src, gfp_mask, bs);
>  		if (!bio)
>  			goto free_and_out;
> +		bio->bi_bdev = rq->q->disk->part0;
>  
>  		if (bio_ctr && bio_ctr(bio, bio_src, data))
>  			goto free_and_out;
> -- 
> 2.30.2

Reviewed-by: Benjamin Marzinski <bmarzins at redhat.com>