On Mon, Jun 04, 2018 at 03:58:53PM +0200, Christoph Hellwig wrote:
> Replace a nasty hack with a different nasty hack to prepare for multipage
> bio_vecs. By moving the temporary page array as far up as possible in
> the space allocated for the bio_vec array we can iterate forward over it
> and thus use bio_add_page. Using bio_add_page means we'll be able to
> merge physically contiguous pages once support for multipath bio_vecs is
> merged.
>
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
> block/bio.c | 45 +++++++++++++++++++++------------------------
> 1 file changed, 21 insertions(+), 24 deletions(-)
>
> diff --git a/block/bio.c b/block/bio.c
> index d95fab72acb5..b09c133a1e24 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -871,6 +871,8 @@ int bio_add_page(struct bio *bio, struct page *page,
> }
> EXPORT_SYMBOL(bio_add_page);
>
> +#define PAGE_PTRS_PER_BVEC (sizeof(struct bio_vec) / sizeof(struct page *))
> +
> /**
> * bio_iov_iter_get_pages - pin user or kernel pages and add them to a bio
> * @bio: bio to add pages to
> @@ -882,38 +884,33 @@ EXPORT_SYMBOL(bio_add_page);
> int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
> {
> unsigned short nr_pages = bio->bi_max_vecs - bio->bi_vcnt;
> + unsigned short entries_left = 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;
> + ssize_t size, left;
> + unsigned len, i;
> + size_t offset;
> +
> + /*
> + * Move page array up in the allocated memory for the bio vecs as
> + * far as possible so that we can start filling biovecs from the
> + * beginning without overwriting the temporary page array.
> + */
> + BUILD_BUG_ON(PAGE_PTRS_PER_BVEC < 2);
> + pages += entries_left * (PAGE_PTRS_PER_BVEC - 1);
>
> size = iov_iter_get_pages(iter, pages, LONG_MAX, nr_pages, &offset);
> 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;
> - }
> + for (left = size, i = 0; left > 0; left -= len, i++) {
> + struct page *page = pages[i];
>
> - bv[0].bv_offset += offset;
> - bv[0].bv_len -= offset;
> - if (diff)
> - bv[bio->bi_vcnt - 1].bv_len -= diff;
> + len = min_t(size_t, PAGE_SIZE - offset, left);
> + if (WARN_ON_ONCE(bio_add_page(bio, page, len, offset) != len))
> + return -EINVAL;
> + offset = 0;
> + }
One invariant is that the page index 'i' is always <= bvec index
of the added page, and the two can only be same when adding the last
page.
So this approach is correct and looks smart & clean:
Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx>
Thanks,
Ming
