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

[Hannes Reinecke <hare@suse.de>]

On 12/6/20 6:18 AM, 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);
>   	}
>   
> 
I don't think this is correct.
This code is submitting the original bio, and we _want_ to keep the 
newly allocated one even though the original might have been completed 
already. If we were setting the 'parent' to the original bio upper 
layers might infer that the entire request has been completed (as the 
original bio is now the 'parent' bio), which is patently not true.

So, rather not.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
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