Re: [PATCH v1 6/6] block/iomap: don't copy bvec for direct IO

[Christoph Hellwig <hch@infradead.org>]

On Tue, Dec 15, 2020 at 12:20:25AM +0000, Pavel Begunkov wrote:
> The block layer spends quite a while in blkdev_direct_IO() to copy and
> initialise bio's bvec. However, if we've already got a bvec in the input
> iterator it might be reused in some cases, i.e. when new
> ITER_BVEC_FLAG_FIXED flag is set. Simple tests show considerable
> performance boost, and it also reduces memory footprint.
> 
> Suggested-by: Matthew Wilcox <willy@infradead.org>
> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> ---
>  Documentation/filesystems/porting.rst |  9 ++++
>  block/bio.c                           | 64 +++++++++++----------------
>  include/linux/bio.h                   |  3 ++
>  3 files changed, 38 insertions(+), 38 deletions(-)
> 
> diff --git a/Documentation/filesystems/porting.rst b/Documentation/filesystems/porting.rst
> index 867036aa90b8..47a622879952 100644
> --- a/Documentation/filesystems/porting.rst
> +++ b/Documentation/filesystems/porting.rst
> @@ -865,3 +865,12 @@ no matter what.  Everything is handled by the caller.
>  
>  clone_private_mount() returns a longterm mount now, so the proper destructor of
>  its result is kern_unmount() or kern_unmount_array().
> +
> +---
> +
> +**mandatory**
> +
> +For bvec based itererators bio_iov_iter_get_pages() now doesn't copy bvecs but
> +uses the one provided. Anyone issuing kiocb-I/O should ensure that the bvec and
> +page references stay until I/O has completed, i.e. until ->ki_complete() has
> +been called or returned with non -EIOCBQUEUED code.
> diff --git a/block/bio.c b/block/bio.c
> index 3192358c411f..f8229be24562 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -960,25 +960,16 @@ void bio_release_pages(struct bio *bio, bool mark_dirty)
>  }
>  EXPORT_SYMBOL_GPL(bio_release_pages);
>  
> +static int bio_iov_bvec_set(struct bio *bio, struct iov_iter *iter)
>  {
> +	WARN_ON_ONCE(BVEC_POOL_IDX(bio) != 0);
> +	bio->bi_vcnt = iter->nr_segs;
> +	bio->bi_max_vecs = iter->nr_segs;
> +	bio->bi_io_vec = (struct bio_vec *)iter->bvec;
> +	bio->bi_iter.bi_bvec_done = iter->iov_offset;
> +	bio->bi_iter.bi_size = iter->count;

Nit: I find an empty liner after WARN_ON_ONCE that assert the caller
state very helpful when reading the code.

>  static inline int bio_iov_vecs_to_alloc(struct iov_iter *iter, int max_segs)
>  {
> +	/* reuse iter->bvec */

Maybe add a ", see bio_iov_bvec_set for details" here?