Re: [PATCH] block: fix bio chaining in blk_next_bio()

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

On Sun, Dec 06, 2020 at 01:18:02PM +0800, Tom Yan wrote:
> While it seems to have worked for so long, it doesn't seem right
> that we set the new bio as the parent. bio_chain() seems to be used
> in the other way everywhere else anyway.
> 
> Signed-off-by: Tom Yan <tom.ty89@gmail.com>
> ---
>  block/blk-lib.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/block/blk-lib.c b/block/blk-lib.c
> index e90614fd8d6a..918deaf5c8a4 100644
> --- a/block/blk-lib.c
> +++ b/block/blk-lib.c
> @@ -15,7 +15,7 @@ struct bio *blk_next_bio(struct bio *bio, unsigned int nr_pages, gfp_t gfp)
>  	struct bio *new = bio_alloc(gfp, nr_pages);
>  
>  	if (bio) {
> -		bio_chain(bio, new);
> +		bio_chain(new, bio);
>  		submit_bio(bio);
>  	}

This patch isn't needed. We simply wait for completion of the last issued bio, which
.bi_end_io(submit_bio_wait_endio) is only called after all previous bios are done.

And the last bio is the ancestor of the whole chained bios, which are
submitted in the following way(order):

	bio 0 ---> bio 1 ---> ... -> bio N(the last bio)


thanks,
Ming