Re: [PATCH 01/34] block: add a lower-level bio_add_page interface

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, May 23, 2018 at 04:43:24PM +0200, Christoph Hellwig wrote:
> For the upcoming removal of buffer heads in XFS we need to keep track of
> the number of outstanding writeback requests per page.  For this we need
> to know if bio_add_page merged a region with the previous bvec or not.
> Instead of adding additional arguments this refactors bio_add_page to
> be implemented using three lower level helpers which users like XFS can
> use directly if they care about the merge decisions.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: Jens Axboe <axboe@kernel.dk>

Looks ok,
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> ---
>  block/bio.c         | 96 +++++++++++++++++++++++++++++----------------
>  include/linux/bio.h |  9 +++++
>  2 files changed, 72 insertions(+), 33 deletions(-)
> 
> diff --git a/block/bio.c b/block/bio.c
> index 53e0f0a1ed94..fdf635d42bbd 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -773,7 +773,7 @@ int bio_add_pc_page(struct request_queue *q, struct bio *bio, struct page
>  			return 0;
>  	}
>  
> -	if (bio->bi_vcnt >= bio->bi_max_vecs)
> +	if (bio_full(bio))
>  		return 0;
>  
>  	/*
> @@ -821,52 +821,82 @@ int bio_add_pc_page(struct request_queue *q, struct bio *bio, struct page
>  EXPORT_SYMBOL(bio_add_pc_page);
>  
>  /**
> - *	bio_add_page	-	attempt to add page to bio
> - *	@bio: destination bio
> - *	@page: page to add
> - *	@len: vec entry length
> - *	@offset: vec entry offset
> + * __bio_try_merge_page - try appending data to an existing bvec.
> + * @bio: destination bio
> + * @page: page to add
> + * @len: length of the data to add
> + * @off: offset of the data in @page
>   *
> - *	Attempt to add a page to the bio_vec maplist. This will only fail
> - *	if either bio->bi_vcnt == bio->bi_max_vecs or it's a cloned bio.
> + * Try to add the data at @page + @off to the last bvec of @bio.  This is a
> + * a useful optimisation for file systems with a block size smaller than the
> + * page size.
> + *
> + * Return %true on success or %false on failure.
>   */
> -int bio_add_page(struct bio *bio, struct page *page,
> -		 unsigned int len, unsigned int offset)
> +bool __bio_try_merge_page(struct bio *bio, struct page *page,
> +		unsigned int len, unsigned int off)
>  {
> -	struct bio_vec *bv;
> -
> -	/*
> -	 * cloned bio must not modify vec list
> -	 */
>  	if (WARN_ON_ONCE(bio_flagged(bio, BIO_CLONED)))
> -		return 0;
> +		return false;
>  
> -	/*
> -	 * For filesystems with a blocksize smaller than the pagesize
> -	 * we will often be called with the same page as last time and
> -	 * a consecutive offset.  Optimize this special case.
> -	 */
>  	if (bio->bi_vcnt > 0) {
> -		bv = &bio->bi_io_vec[bio->bi_vcnt - 1];
> +		struct bio_vec *bv = &bio->bi_io_vec[bio->bi_vcnt - 1];
>  
> -		if (page == bv->bv_page &&
> -		    offset == bv->bv_offset + bv->bv_len) {
> +		if (page == bv->bv_page && off == bv->bv_offset + bv->bv_len) {
>  			bv->bv_len += len;
> -			goto done;
> +			bio->bi_iter.bi_size += len;
> +			return true;
>  		}
>  	}
> +	return false;
> +}
> +EXPORT_SYMBOL_GPL(__bio_try_merge_page);
>  
> -	if (bio->bi_vcnt >= bio->bi_max_vecs)
> -		return 0;
> +/**
> + * __bio_add_page - add page to a bio in a new segment
> + * @bio: destination bio
> + * @page: page to add
> + * @len: length of the data to add
> + * @off: offset of the data in @page
> + *
> + * Add the data at @page + @off to @bio as a new bvec.  The caller must ensure
> + * that @bio has space for another bvec.
> + */
> +void __bio_add_page(struct bio *bio, struct page *page,
> +		unsigned int len, unsigned int off)
> +{
> +	struct bio_vec *bv = &bio->bi_io_vec[bio->bi_vcnt];
>  
> -	bv		= &bio->bi_io_vec[bio->bi_vcnt];
> -	bv->bv_page	= page;
> -	bv->bv_len	= len;
> -	bv->bv_offset	= offset;
> +	WARN_ON_ONCE(bio_flagged(bio, BIO_CLONED));
> +	WARN_ON_ONCE(bio_full(bio));
> +
> +	bv->bv_page = page;
> +	bv->bv_offset = off;
> +	bv->bv_len = len;
>  
> -	bio->bi_vcnt++;
> -done:
>  	bio->bi_iter.bi_size += len;
> +	bio->bi_vcnt++;
> +}
> +EXPORT_SYMBOL_GPL(__bio_add_page);
> +
> +/**
> + *	bio_add_page	-	attempt to add page to bio
> + *	@bio: destination bio
> + *	@page: page to add
> + *	@len: vec entry length
> + *	@offset: vec entry offset
> + *
> + *	Attempt to add a page to the bio_vec maplist. This will only fail
> + *	if either bio->bi_vcnt == bio->bi_max_vecs or it's a cloned bio.
> + */
> +int bio_add_page(struct bio *bio, struct page *page,
> +		 unsigned int len, unsigned int offset)
> +{
> +	if (!__bio_try_merge_page(bio, page, len, offset)) {
> +		if (bio_full(bio))
> +			return 0;
> +		__bio_add_page(bio, page, len, offset);
> +	}
>  	return len;
>  }
>  EXPORT_SYMBOL(bio_add_page);
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index ce547a25e8ae..3e73c8bc25ea 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -123,6 +123,11 @@ static inline void *bio_data(struct bio *bio)
>  	return NULL;
>  }
>  
> +static inline bool bio_full(struct bio *bio)
> +{
> +	return bio->bi_vcnt >= bio->bi_max_vecs;
> +}
> +
>  /*
>   * will die
>   */
> @@ -470,6 +475,10 @@ void bio_chain(struct bio *, struct bio *);
>  extern int bio_add_page(struct bio *, struct page *, unsigned int,unsigned int);
>  extern int bio_add_pc_page(struct request_queue *, struct bio *, struct page *,
>  			   unsigned int, unsigned int);
> +bool __bio_try_merge_page(struct bio *bio, struct page *page,
> +		unsigned int len, unsigned int off);
> +void __bio_add_page(struct bio *bio, struct page *page,
> +		unsigned int len, unsigned int off);
>  int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter);
>  struct rq_map_data;
>  extern struct bio *bio_map_user_iov(struct request_queue *,
> -- 
> 2.17.0
> 
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