Re: [PATCH 3/3] block: use bio_add_page in bio_iov_iter_get_pages

[Ming Lei <ming.lei@redhat.com>]

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@lst.de>
> ---
>  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@redhat.com>

Thanks,
Ming