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SubjectRe: [PATCH 1/2] block: add bio_iov_iter_get_pages()
On Tue, Nov 1, 2016 at 1:59 AM, Christoph Hellwig <hch@lst.de> wrote:
> From: Kent Overstreet <kent.overstreet@gmail.com>
>
> 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@gmail.com>
> [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@lst.de>
> ---
> 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

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