Re: [PATCH 1/2] block: add bio_iov_iter_get_pages()

[Ming Lei]

On Tue, Nov 1, 2016 at 1:59 AM, Christoph Hellwig <hch@xxxxxx> wrote:
> From: Kent Overstreet <kent.overstreet@xxxxxxxxx>
>
> This is a helper that pins down a range from an iov_iter and adds it to
> a bio without requiring a separate memory allocation for the page array.
> It will be used for upcoming direct I/O implementations for block devices
> and iomap based file systems.
>
> Signed-off-by: Kent Overstreet <kent.overstreet@xxxxxxxxx>
> [hch: ported to the iov_iter interface, renamed and added comments.
>       All blame should be directed to me and all fame should go to Kent
>       after this!]
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
>  block/bio.c         | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/bio.h |  1 +
>  2 files changed, 50 insertions(+)
>
> diff --git a/block/bio.c b/block/bio.c
> index db85c57..2cf6eba 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -847,6 +847,55 @@ int bio_add_page(struct bio *bio, struct page *page,
>  }
>  EXPORT_SYMBOL(bio_add_page);
>
> +/**
> + * bio_iov_iter_get_pages - pin user or kernel pages and add them to a bio
> + * @bio: bio to add pages to
> + * @iter: iov iterator describing the region to be mapped
> + *
> + * Pins as many pages from *iter and appends them to @bio's bvec array. The
> + * pages will have to be released using put_page() when done.
> + */
> +int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
> +{
> +       unsigned short nr_pages = bio->bi_max_vecs - bio->bi_vcnt;
> +       struct bio_vec *bv = bio->bi_io_vec + bio->bi_vcnt;
> +       struct page **pages = (struct page **)bv;
> +       size_t offset, diff;
> +       ssize_t size;
> +
> +       size = iov_iter_get_pages(iter, pages, LONG_MAX, nr_pages, &offset);

BTW, if there is one multi-page version of get_user_pages_fast() and
iov_iter_get_pages(), size of page array can be reduced too.

> +       if (unlikely(size <= 0))
> +               return size ? size : -EFAULT;
> +       nr_pages = (size + offset + PAGE_SIZE - 1) / PAGE_SIZE;
> +
> +       /*
> +        * Deep magic below:  We need to walk the pinned pages backwards
> +        * because we are abusing the space allocated for the bio_vecs
> +        * for the page array.  Because the bio_vecs are larger than the
> +        * page pointers by definition this will always work.  But it also
> +        * means we can't use bio_add_page, so any changes to it's semantics
> +        * need to be reflected here as well.
> +        */
> +       bio->bi_iter.bi_size += size;
> +       bio->bi_vcnt += nr_pages;
> +
> +       diff = (nr_pages * PAGE_SIZE - offset) - size;
> +       while (nr_pages--) {
> +               bv[nr_pages].bv_page = pages[nr_pages];
> +               bv[nr_pages].bv_len = PAGE_SIZE;
> +               bv[nr_pages].bv_offset = 0;
> +       }
> +
> +       bv[0].bv_offset += offset;
> +       bv[0].bv_len -= offset;
> +       if (diff)
> +               bv[bio->bi_vcnt - 1].bv_len -= diff;
> +
> +       iov_iter_advance(iter, size);
> +       return 0;
> +}
> +EXPORT_SYMBOL_GPL(bio_iov_iter_get_pages);
> +
>  struct submit_bio_ret {
>         struct completion event;
>         int error;
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index 87ce64d..c39fa0b 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -419,6 +419,7 @@ 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);
> +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 *,
>                                     const struct iov_iter *, gfp_t);
> --
> 2.1.4
>
> --
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-- 
Ming Lei