Re: [PATCH 1/6] block: add a bio_reuse helper

[Kent Overstreet <kent.overstreet@gmail.com>]

On Mon, Jun 11, 2018 at 09:48:01PM +0200, Christoph Hellwig wrote:
> This abstracts out a way to reuse a bio without destroying the
> data pointers.

What is the point of this? What "data pointers" does it not destroy?

> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  block/bio.c         | 20 ++++++++++++++++++++
>  include/linux/bio.h |  1 +
>  2 files changed, 21 insertions(+)
> 
> diff --git a/block/bio.c b/block/bio.c
> index 70c4e1b6dd45..fa1b7ab50784 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -308,6 +308,26 @@ void bio_reset(struct bio *bio)
>  }
>  EXPORT_SYMBOL(bio_reset);
>  
> +/**
> + * bio_reuse - prepare a bio for reuse
> + * @bio:	bio to reuse
> + *
> + * Prepares an already setup and possible used bio for reusing it another
> + * time.  Compared to bio_reset() this preserves the bio size and the
> + * layout and contents of the bio vectors.
> + */
> +void bio_reuse(struct bio *bio)
> +{
> +	unsigned int size = bio->bi_iter.bi_size;
> +	unsigned short vcnt = bio->bi_vcnt;
> +
> +	bio_reset(bio);
> +
> +	bio->bi_iter.bi_size = size;
> +	bio->bi_vcnt = vcnt;
> +}
> +EXPORT_SYMBOL_GPL(bio_reuse);
> +
>  static struct bio *__bio_chain_endio(struct bio *bio)
>  {
>  	struct bio *parent = bio->bi_private;
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index f08f5fe7bd08..15c871ab50db 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -475,6 +475,7 @@ extern void bio_init(struct bio *bio, struct bio_vec *table,
>  		     unsigned short max_vecs);
>  extern void bio_uninit(struct bio *);
>  extern void bio_reset(struct bio *);
> +void bio_reuse(struct bio *);
>  void bio_chain(struct bio *, struct bio *);
>  
>  extern int bio_add_page(struct bio *, struct page *, unsigned int,unsigned int);
> -- 
> 2.17.1
> 
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